Catherine Halsey

"For a long time I had thought that we had to sacrifice a few for the good of the entire human race. I have killed and maimed and caused a great deal of suffering to many people—all in the name of self-preservation. But now I'm not sure that philosophy has worked out too well. I should have been trying to save every single human life—no matter what it cost."

- Catherine Halsey to John-117 regarding her changed views.

Doctor Catherine Elizabeth Halsey, (Civilian Consultant 409871, phonetically identified as "Charlie Hotel"), is a key scientific adviser of the Office of Naval Intelligence, best known for her work with the SPARTAN-II program, developing and creating smart AIs, and for overseeing the development of the MJOLNIR Powered Assault Armor. She served as a scientific adviser for ONI from 2515 to 2522, when she was made chief scientist, a position she held until the end of the Human-Covenant War.

Halsey held considerable authority within ONI, despite being a civilian, and commanded respect from important military figures. Though generally well-regarded among her peers, she endured intense personal guilt for condemning the Spartans to the life of a super soldier. After the Covenant War, Dr. Halsey was stripped of her position, incarcerated, and treated as a war criminal by the UNSC at large, which led to her apparent defection to Jul 'Mdama's splinter Covenant faction. After almost a year among the enemy, she was recaptured by the UNSC and commissioned to work on urgent scientific matters, particularly concerning the rise of the Guardians.

Early life and education
"The Office of Naval Intelligence had apparently had their eye on me for years and knew that the only way to convince me to join them was to get me to convince me. It worked. I joined."

- Catherine Halsey recalls her recruitment by ONI in her journal. Catherine Elizabeth Halsey was born on March 19, 2492 within the colony of Endymion's coastal city of Port Vernon. While an impressionable young child her father recited to her every adventurous tale of L. Frank Baum's Oz At this age she was already contemplating ideas having to do with artificial intelligences and their longevity. As an adolescent she was smarter than her parents and was continually reading, learning, and talking, being eager to share her knowledge with anyone who would listen. The first game of chess Catherine played with her mother was the only one she ever lost to her. Upon beating her daughter she had said, in Italian, "When the game is over, the king and pawn go into the same box." At a young age, Halsey was sent to the elite academy Endymion Gifted, where she continued to defy educatory standards. She was eventually afforded a trip to Circumstance in 2501 from the Dean of Biological Science at Koletre-Browning University, who completely funded her education until the completion of her second doctoral thesis sometime after her fifteenth birthday in 2507. While working on that second thesis, Halsey learned various line commands, long discarded and forgotten due to their archaic nature, with which she could interact with computer programs.

In 2507, while still a doctoral candidate, Halsey met Doctor Elias Carver at an academic social mixer and couldn't resist correcting his obsolete algorithm implementation when she heard him describing the matrix mechanics of the socio-economic and politico-economic vectors of human expansion. She outlined a corrective matrix calculation to him which revised his dimensional parameters so that the sixteen dimensions were used rather than seven. Doctor Carver did not appreciate being challenged by someone he saw as a young upstart, but her words seemingly caused Captain Michael Stanforth of the United Nations Space Command's Office of Naval Intelligence to take note of her. Three weeks later, a grant she had proposed for using artificial intelligences to control N-dimensional matrices was green-lit by the UNSC, with its funding contingent on the successful testing of a model of its choosing. Naturally, it was Carver's model which she was directed to test. Halsey knew his flawed model would produce flawed results, so she ran her sixteen dimensional variant as a control in addition to his seven-dimensional version. Carver's model predicted a breakdown of social order in the Outer Colonies within twenty years unless strict governmental control was established, reinforced by an immediate and permanent military presence. Catherine ran more then fourteen hundred simulations during this time, varying every parameter, and even in the best-case scenario it was clear to her that the Outer Colonies at large would rebel against the Unified Earth Government soon. She determined that at a minimum, the resulting conflict would lead to thirty years of war and the deaths of five billion people. The maximum effect was unbounded and could involve interminable war and a new dark age for humanity. Halsey took her findings to Stanforth only to learn that ONI had already reached the same conclusion. The agency had been keeping an eye on her for years and it had been decided that in order to recruit her, she would need to convince herself that working with the agency was necessary to save the human race. Ysionris Jeromi, who considered Halsey his star pupil at the time, discouraged her from working with ONI. Halsey, however, had no problem admitting that ONI's tactic had worked on her. She allowed herself to be recruited in 2508 and in early 2510 she was brought to the Inner Colony of Reach and contracted to work as a civilian consultant on a number of classified projects.

Spartan origins
"There is an ancient saying, "The beginning of wisdom is ignorance." So where do we begin to create the ultimate warrior? With innocence."

- Halsey, writing in her journal, appropriates a maxim oft attributed to Socrates for her own ends. After arriving at Reach, Doctor Halsey spent a few months setting up the new laboratory afforded to her. For weeks prior to August 8, 2510 she found it impossible to concentrate due to the confluence of new insurgent reports, new ideas, and the constant supervision of ONI. At one point during this time she stopped at a local shop while coming from the main base where she was stationed and purchased a cup of coffee and a journal in which she could record her thoughts and research notes without having to fear them being read by anyone in ONI. By August 8, the day on which she recorded her first entry in this journal, she had begun to undertake a project she had been simultaneously dreading, desiring, and being inspired by. By this point Catherine had already designed a number of intrusion algorithms that could be employed to bypass artificial intelligence-enhanced data recording and storage systems. By December 4 of that year Halsey's new lab was still unstaffed apart from herself and racks of A.I. matrices and optical routers which were yet to be removed from their sealed crates. While sitting in the lab on this date Halsey endeavored to create her second entry in her journal, recalling the series of events which had led her to Reach and to be working for ONI. She ended the entry by asking how many lives would need to be spent to save all humanity and whether any price was too high to pay in order to ensure its security, resuming her work thereafter. Evidently to aid in this endeavor, she also made herself a reminder to acquire more dark coffee. On February 15, 2511, Doctor Halsey in her journal went over details of the ORION Project and recorded ways in which she believed its successor could improve on what it had accomplished. ORION had been designed to create a nontraditional force of soldiers in order to remove the budding dissident leadership in the Outer Colonies without massive carnage, and to this end volunteers were recruited from the UNSC's Special Forces. Based on the failures documented surrounding the genetic augmentation and immunosuppressant administration provided to ORION candidates, Halsey determined that the most suitable candidates fur a future generation of augmented warriors would need to be prepubescent and possess more malleable and robust DNA structures and repair enzymes. In order to locate individuals with the improbable genetic criteria required, she calculated that thirty-nine billion DNA records would need to be screened. Halsey realized the Colonial Administration Authority's Outer Colony vaccination program possessed the largest DNA database at that time and resolved to recalibrate her selection criteria to find markers which would expedite the sifting process. Additionally, she recognized that in order to mitigate the posttraumatic stress disorder and sympathy for the insurgents exhibited by many from ORION, total indoctrination would be required. She was opposed to the sort of brainwashing several of her counterparts in the intelligence community had suggested, noting instead that the mission of "Generation-II ORION" demanded at least a decade of training, persuasion and acclimation. Above all, she wrote, absolute control must be maintained over those chosen. It was clear to Halsey, based on these parameters, that children were ideal for this project. The ethical and moral implications of such an idea forced her to take extra time to think about this, however. Illustrations drawn in her journal on this date outlined her considerations of various human augmentations suitable for the next generation of the ORION Project, which she also was considering renaming something more inspirational on behalf of her military counterparts. Just over a week later, on February 23, Halsey read the news about a nuclear terror attack at the Haven arcology on Mamore which left two million dead and millions more expected to die from the fallout. It was reported that the Freedom and Liberation Party was claiming responsibility and upon learning this Halsey became even more convinced than she already had been that she had to stop the violence in the Outer Colonies from escalating further.

On July 30, 2511 Halsey received test results for a new polymerized lithium niobocene. The material had been developed for discharging the vast amount of static energy that plagued ships in slipstream space but Halsey was interested in its potential applications as a sheath or suit of artificial muscle for a Gen-II ORION soldier. Though manufacturing an entire suit with it was cost-prohibitive at that point, Halsey mentioned in her journal on this date that Stanforth, now a vice admiral and the head of ONI Section Three, had assured her this would be addressed in the "production phase" of the project. Given the scope, Halsey remained doubtful. Additionally, although she believed there was a way to couple the polymerized lithium niobocene to a wearer's nerve inductions to increase reaction time and had created notes on the subject and stored them within the A.I. Jorjet, she had yet to solve the matter of the energy required for such a design. She'd concluded a small nuclear or fusion reactor would need to be attached to the suit, which would be unwieldy, dangerous, and expensive. Halsey and others agreed that the concept behind the Navy's original Mark I prototype powered exoskeletons needed to be scrapped for Project: MJOLNIR. Needing to be plugged into a generator or to rely on broadcast power made them wholly inefficient for battlefield use. Looking over the Mark I's antiquated design at this time, Halsey still wondered how much of it could remain useful.

On September 8, 2511 Halsey recorded in her journal that she had recently finalized the control parameters for a Fast Fourier transform X-ray 3D scan for mapping a brain's ionic density patterns that indicate individual links. This meant that a deep stimulating scan could then cultivate growth linkages in a cloned brain, thereby allowing memories to be transferred for the first time. Complete human clones could feasibly now be produced that were indistinguishable from the individual they were based on. The potential application of this breakthrough for ORION-II was obvious, and Halsey was of the mind that stealing children away and replacing them with clones would mean there would be no going back. By the time she updated her journal with this, she had already informed the vice admiral that they were ready to initiate the next phase of the project: candidate procurement. She had done this knowing that only a small fraction of the clones they produced could be expected to live a normal lifespan due to the amount of DNA base-pair errors rapid flash cloning of large volumes of tissue produced. Beyond what this breakthrough meant for her current project, Halsey also speculated on whether this technique could imprint the next-generation of artificial intelligences and in so doing avoid the wavefunction collapse that had plagued the functionality of previous generations.

On June 19, 2513, Doctor Halsey settled on a new name for the project. Stanforth had seemed to hint that rechristening the project would serve to distance it from the failures of ORION. After having Jorjet and Déjà (an A.I. created to assist her with the program) give her a refresher on military history which produced a number of contenders, she decided "SPARTAN" best represented the project's aims. The Spartans of Ancient Greece had been known for their legendary training of young boys and eugenic customs, and the defense of Greece made at Thermopylae was a well-known and cherished tale within the military community. Coincidentally, Halsey did not fail to note, her project had originally been funded for three hundred candidates. In order to honor the sacrifices made by the men and women of the original ORION effort, she opted to call the project "SPARTAN-II", preserving its status as the second iteration of that first effort. In 2515, Halsey was officially named a scientific advisor for ONI.

Candidate selection
"Catherine Halsey"

- Jacob Keyes By July 21, 2517, Halsey had been provided with her own team, to which she delegated much of the responsibly for screening Outer Colonies candidates for SPARTAN-II. She did this so it might be completed within a reasonable time frame and so that she could personally focus on processing those from a single region of space: Sector 4. The children from this sector which met the criteria Halsey had identified in her original study had files assembled personally by her. In total, one hundred and fifty subjects were to be considered. The Spartan-II program had only been granted facilities and funding for half that number, however, so only seventy-five would ultimately be selected. And even then, an ONI representative reminded Halsey that not all would make it through the augmentation procedures when the time came for that. Vice Admiral Stanforth had agreed to lend Halsey a diplomatic shuttle called the Han, provided that she agreed to at least one naval officer accompanying her. Halsey was given free reign to choose her attaché and after reading through the file of one Lieutenant Junior Grade Jacob Keyes, she requested him based on his demonstrated ability to keep a secret. Keyes was then, without any explanation provided to him as to why, assigned to pilot the Han, protect the doctor, and otherwise stay out of her way. On July 21 Halsey wrote in her journal that she was considering recommending him for permanent reassignment to her staff. The Han also came with its own dedicated artificial intelligence by the name of Toran.

Halsey was awakened from cryostasis alongside Keyes shortly after 0430 hours on August 17 while the Han was still in slipspace but set to emerge soon in the Eridanus system. Giving not so much as an acknowledgement that they were both naked, Halsey simply climbed out of her tube and told the lieutenant to get cleaned up and dressed as she headed for the showers herself, making it clear to Keyes that she was in charge. Halsey was already strapped into the navigator's couch and on the bridge and tapping in commands across four keypads when he joined her soon thereafter. The doctor welcomed him and asked that he be seated at the communication station to monitor channels when they reentered normal space. She then had Toran provide her with astronavigation maps of the system and asked if any planets between their current position and Eridanus II could be used to pick up a gravitational boost to let them arrive sooner. Additionally, in rapid succession she had the A.I. begin to play Rachmaninov's Piano Concerto Number Three, begin a preburn warmup cycle for their fusion engines, and halt the rotation of the shuttle's central carousel section to conserve power. Toran obliged with each request and informed her they they would exiting slipspace in about five minutes, give or take three minutes. From the way Keyes was glancing at her, Halsey could tell he was impressed at her ability to put Toran through his paces so rigorously, so she asked if he had a question for her. In response he composed himself before inquiring as to the exactly what they were doing. Halsey told him that they were on a reconnaissance mission of sorts, as they were heading to Eridanus II in order to observe a child whom she hoped would only be the first of many. She suggested the junior lieutenant should just think of it as a UNSC-funded psychological study and then sternly told him that was exactly what he was to say it was to anyone who asked. The Han soon decelerated as it dropped out of slipspace near a gas giant and then began a hard burn, accelerating for sixty-seven seconds before arcing away from the planet. As soon as the boost was complete Toran informed the two humans that their ETA was forty-three minutes and three seconds. At this, Halsey unlocked her harness and floated free. Just before they had dropped into normal space, Keyes had started to ask her if she knew how dangerous the Eridanus system was then and now she finally answered him in the affirmative. After stretching and strapping herself back into her couch, she reminded her escort that he was supposed to be monitoring their comms for that very reason. Catherine spent the next thirty minutes with her back to Keyes while she read over reports on her navigation screens. The Keyes eventually spoke up to candidly ask why he was there, and Halsey told him the truth about why she had selected him for the job. She also assured him that though he may have rather been on the then, which now-rescinded orders had previously had him rotating to, the child they were traveling to observe could make all the difference for the ongoing struggle against the Insurrection. Once Toran announced they were approaching Eridanus II Halsey had him plot an atmospheric vector for the Luxor spaceport and ordered Keyes to make ready to land. Halsey and Keyes made their way onto the campus of Elysium City's Primary Education Facility Number 119 under the guise of parents inspecting the school for their little girl. By 1130 hours they were watching children at play from the semi-shade of a canvas awning. Halsey, wearing a sundress and straw hat, urged the lieutenant to appear more relaxed and even tried to help by slipping her arm through his, but her efforts were in vain. It was Keyes, however, who first identified their subject, a six-year-old boy named John, after Halsey showed an image to him. John was atop a grassy hill at the end of the school's playground, engaged in a particularly vicious game of king of the hill. Halsey's first instinct was to point her data pad at the boy to record the incident for later study. After glancing around to see that the only other adult present was marching an injured girl toward the nurse's office Halsey ordered Keyes to stay back and keep watch while she walked over to get a closer look at their boy. She stopped at four meters from the base of the hill and asked to speak with John, whom the other boys assumed must be in trouble. She led him to the edge of a nearby sandpit and asked his name, which he freely offered before holding out his hand to be shook. She asked what he was had been doing and John answered that he had been winning. Seeing that the boy was fond of games, she produced a quarter and told him that if he could guess which side of the coin would be facing upwards when it landed after being flipped, he could keep it. She let it go but before it could hit the ground John snatched it out of the air and confidently declared "Eagle!" referring to the side of the quarter which was, indeed, facing up when he opened his hand. Assured he could keep the coin for himself, he asked if they could play again but Halsey told him that had been the only one she had. She told him he ought to go back to his friends and returned to the lieutenant, asking to make sure he had recorded the interaction. He had, and after reviewing the footage Halsey sent it to Toran on the Han. She explained to Keyes that the one thing they couldn't test their candidates for remotely was luck, further clarifying that she meant her test as a simple way to eliminate half of the qualified children due to the budgetary constraints of the program. She remarked that John whether John was lucky or just extraordinarily fast, he had made the cut. Keyes still did not fully understand her interest in the child and said as much, to which she replied that she hoped he never would fully understand their purpose there.

After similar visits to multiple other children in Sector Four, The Han took them to Dwarka, a harsh world still in the process of being terraformed. The child to be considered there was an orphan by the name of Soren. The report Halsey had previously read indicated the boy had survived on his own for around three months following the deaths of his mother and stepfather. Halsey left Keyes aboard the Han and approached Soren alone at the place he was being cared for by this point. She asked him his name, but admitted she already knew what it was and only wanted to see if he'd be willing to tell her. He had not been, evidently being deeply distrustful of the doctor. A pair of glasses Halsey wore allowed her to review holographic files on the lenses as she spoke to the boy, and as a result of his circumstances she chose to pursue a different strategy. She was as honest with him insofar as she told him she'd come to see him because she wanted to make him stronger, faster, and smarter than he could ever hope to be without her help. She also made sure he understood that if he chose to accept her offer, he would be faced with the hardest experiences he had yet―even harder than what had happened after his parents died. Faced with the choice of leaving with Doctor Halsey or being placed in a foster home, he quickly agreed to go with her. Halsey was able to convince the authorities that she was adopting the boy and bring him aboard the Han. There she showed Keyes the footage she'd recorded of their conversation. The lieutenant was uneasy with her decision to take then child then. Halsey was sure that Soren would be a good fit, however, and defended his acquisition by pointing out at least he had been warned concerning what lay ahead.

SPARTAN-II


Halsey and Keyes had returned to Reach by September 10 and it is on this date that Halsey told Déjà to let others know she had settled on the final seventy-five subjects. Halsey was stationed in low Reach orbit when she gave final approval to kidnap the seventy-four which still remained to be acquired. Halsey later recorded a private spoken journal entry, using RED encryption and her private key, in which she voiced how regrettable she felt what she was doing was. She had planned to keep the recording private but changed her mind and just had Déjà erase it entirely once she had finished. On September 15 Halsey, writing in her journal, expressed that every child had exceeded the selection criteria and her own expectations. She was particularly impressed by two: Number 058, a girl named Linda who had engineered her own intelligence network at school to spy on her teachers, and John (Number 117), who had won king of the hill forty-five times in the two weeks before their meeting. Halsey noted that Number 095, a boy by the name of Caleb, had never been located. Later that day Halsey added an addendum to her journal entry after Retrieval team Gamma reported that Number 087, a girl named Kelly, had remarkably eluded capture for six hours before coming forward on her own, having believed her capture attempt to be an elaborate game for her upcoming birthday. Gamma's report caused Halsey to ensure new retrieval protocols were implemented to prevent similar mistakes in the future. The next day, September 16, Catherine and Jacob Keyes finally parted company after months together. Halsey felt he was beginning to suspect there was more to their incursions than mere field observations so she requested that he be reassigned to the Magellan with a commission to full lieutenant both for his own protection and the protection of the program. Taken with Keyes, she recalled in her journal on this date an occasion which he had gotten a job done himself rather than waiting for the assistance of a technical crew. Despite saying their goodbyes on the sixteenth, Halsey and Keyes briefly met once more the following day before parting for good. Once all the children had been collected Déjà undertook a psychological evaluation of each and provided the results and her recommendations to Doctor Halsey. By 2300 hours on September 23 Halsey was standing on a platform at the center of an amphitheater far beneath the FLEETCOM military complex where her facilities were located. When Déjà asked if she was ready to speak to the abducted children she told the A.I. she almost was but wished for her to summon Chief Petty Officer Franklin Mendez, the drill instructor who would be overseeing the trainees' physical training and hand-to-hand combat, to join her first. She directed Mendez to join her on the platform once he entered. Déjà asked the doctor for her thoughts on the candidates' psychological evaluations and Halsey, while thanking her for their thoroughness, informed her she was forgoing her recommendation to lie to the children. Halsey argued that though the truth had its own risks, lying to the children about the circumstances of their kidnappings or even inducing memory loss could jeopardize the program's goals. Before Déjà could offer a response Halsey clicked her microphone and gave the command to bring the abductees in. Once all seventy-five were seated, she explained that per Naval Code 45812, they were thereby conscripted into SPARTAN-II. She said that they were to be the UEG's protectors and promised that here they would become the best they could be. She also informed them that they could not return to their homes or families. She closed by telling them to rest then, since their training would begin the next day. Halsey then instructed CPO Mendez to escort the trainees to their barracks to be fed and put to bed, also advising that they be kept plenty busy the following day in order to keep them from thinking about what had just been done to them. Before all the children could file out of the amphitheater one boy asked Halsey if they were there to stop everyone from fighting and killing each other. Before the day was done Halsey noted in her journal that the first phase of the program's indoctrination was under way and that her team of ONI child psychologists was assuring her the children wouldn't even remember their names or families after just six months under Mendez' tutelage. Even so, she also wrote that she was working with Déjà to continue indoctrination protocols and the candidates' education. Before falling asleep Halsey nursed a glass of wine in bed while she viewed reporting on the Insurrection. The news of an escalating death toll reinforced what she doing to the children and she said as much to Déjà before having the A.I. show her footage of the children being fed. On the fifth day of training, the children were given haircuts. Halsey took note in her journal that Kelly-087 had clawed at and disarmed the master sergeant barber of his clippers before needing to be pinned down by three other attendants.

On December 24 Doctor Halsey received a batch of after-action reports regarding the fates of the clones used to replace most of the stolen children. Overall, the clones' longevity had outpaced what laboratory experiments had predicted and Halsey wondered if this could have been owed to the efforts of parents to nurture them back to health. She assigned JACKBOOT to track this further. Writing in her journal, she expressed remorse for her inability to detach herself emotionally from the reports of suffering, death, and grieving. Halsey regularly observed the children from a distance and scribbled notes down during their exercises, with Soren sometimes noticing her watching him in particular. By December 10 of 2518, she and CPO Mendez had identified Kurt-051, Jerome-092, Fred-104, and John-117 as potential leaders for all the trainees. Though Chief Mendez wanted to wager on Kurt, she was confident John would be their leader eventually, something she had been convinced of since the day she'd met him. Halsey was also pleased that they had not yet witnessed the sort of aggressive alpha dominance predicted by ONI's specialists in any of the children.

Work continues
"I've instructed Déjà to emphasize the history of military training—that to simulate combat conditions, lethal force is often employed; that accidents are an inherent risk... perhaps even inevitable. Mendez spoke with the child. She understands. She will recover. This bodes well. There will be many deaths before boot is over."

- Doctor Halsey considers the cost of the Spartan-II training and expresses optimism about the trainees' psychological hardiness. One day, around five months into training, Doctor Halsey asked to see two children who had made repeated attempts to escape the program: Jai-006 and Adriana-111. When Chief Mendez brought Jai into her office she at first pretended he had been the one to initiate the visit before casually asking if he wanted to leave. The boy responded enthusiastically that he did, so she asked him to recall what she'd said to them all when they'd first arrived. Nodding regretfully at his clear recollection and persistent defiance, she then drew his attention to a small dart which she claimed could induce a selective neural paralysis and cause him to forget all of his experiences on Reach so far. She said that should he break out of the forest surrounding the complex again, he would be shot in the head with one of these darts and wake up in a city with no memory of what had happened, provided it didn't do further damage to him. On the other hand, she advised, he could continue his training under people who cared about what happened to him. To contrast his new life with his old, she told him that when the orphanage on Bhuj he'd been housed in was called nobody there had even noticed he'd been missing since she had transferred him. Not even Bhuj's vendors, she'd claimed, remembered him. She finished by exploiting his blossoming friendship with Adriana and making him understand that she could also make her forget about him. Halsey told him that this was her final offer: if he tried to escape that night what she'd said about erasing memories would come to pass. If he stayed, conversely, she offered him a family and a chance to be remembered. Adriana's similar meeting with her left the girl in tears later that evening and unwilling to tell Jai what the doctor had said to her.

On July 12, 2519 the trainees were scattered throughout a military wilderness training preserve in the Highland Mountains and left to find their own way to the extraction point, with the stipulation that the last to arrive would have to walk back to the base. John stubbornly refused to accept those parameters and led the children to steal the D96-TCE Albatross from the guards posted at the extraction point and, with help from Déjà, fly them all back. At around 2104 hours Halsey met John and Mendez in the latter's office, greeting the boy as she seated herself across from the Chief. Once Mendez has finished questioning John regarding his actions, he looked to Halsey for advice regarding what ought to be done with him. At this, she offered that it was obvious to her he should be made squad leader over the rest. On September 20, 2520, the trainees experienced their first casualty in the form of a handler who was accidentally killed by Carris-137. Catherine saw Carris crying outside the barracks following this but decided to leave her alone. She and Chief Mendez both agreed that no disciplinary action was necessary. Memorial services were held with all candidates in attendance and training resumed thereafter. Halsey had Déjà emphasize to the children that throughout the history of military training lethal force had often been employed and that deaths were an inherent risk. Sure that Carris would recover after speaking with Mendez, Halsey chronicled her thoughts on the matter. She thought the girl's response boded well, since it seemed certain there would be many more deaths before boot camp was finished. On October 15, 2521, while enjoying a slice of custard pie from Havadi's, Halsey took the time to muse somewhat in her journal on the vulnerabilities created by the standardization of operating systems, highlighting the recent disabling of a patrolling destroyer by a terrorist who hacked its code. Given that the UNSC, conversely, had not had much success penetrating the homegrown collections of software employed in the Outer Colonies, she concluded that the time had come to think outside the box as far as programming practices were concerned. She had spared no resource in the development of routines that allowed the newly-operational third-generation A.I.s to penetrate any system software. Additionally, they had already learned to mutate and defend established UNSC operating systems from attack, a side effect of the routines which Halsey found serendipitous. Before she concluded writing that day, she took note of the frustration she felt at the way the acronyms for illegal entry protocols and counter-illegal entry protocols had been commonly misspelled and then vocalized based on those misspellings. On June 5, 2522 Halsey was finally offered the position of chief scientist within ONI, after years of only officially serving in an advisory role involving a series of undocumented but well-paid contracts. Since she had always lacked the time or interest to spend the money earned, these contracts had always benefited her little. Halsey accepted the offer, but was uncertain why it had been presented to her then. In her journal she theorized that the promotion might have something do to with the curiosity of her colleagues and the wider scientific community or might simply reflect suspisions that her temporary unofficial advisory role was in reality none of those things. Once around this time, when Soren-066 was nearly eleven years old, she approached him as he ran through an exercise with the other children. Soren had been hesitating, unsure of which team to join, so she asked if everything was alright. The boy replied in the affirmative, but slipped up and called her "Sir" before blushing and correcting his mistake. Halsey just smiled and told him he shouldn't get distracted by irrelevant data, then pointed past him as both teams ran for the skirmish ground and added that above all he shouldn't let himself get left behind.

On January 18, 2524 CFHT-Mars captured a ten minute exposure image of an anomalous object within the star cluster NGC 2244 and nearing human space. An acquaintance of Halsey's later sent her the image for evaluation along with a note describing the unidentified object as having guided movement and not belonging to the UNSC. They also gave her a way to reach them on ChatterNet, should she wish to weigh in on this mystery. August 22, 2524 left Halsey furious after she learned that her candidates had been rerouted during a training exercise to a Special Forces camp on the other side of Reach. Mendez had told her that "interested parties" within ONI wished to observe the teens in action firsthand, which made her immediately wonder about someone trying to usurp her project or cut its budget. She took her grievances to Admiral Stanforth, who claimed to know nothing about what had happened and told her to stop being paranoid. Convinced he was not being truthful and that something was going on without her knowledge, she requested that the production of the next-generation A.I. matrix strata be expedited so her new intrusion and counter-intrusion software could be developed to aid the Spartans and protect herself. On November 17 of that same year, Catherine made an entry in her journal following a conference she had attended at the University of Calippus. While at the conference, she had reunited with Jacob Keyes and had sex with him. She was unsure whether her decision to do so had been a result of the dizzying pressure she was under, a response to a chance encounter with a kindred spirit in a strange place, or something else, but she was glad it had been him she'd run into. The romantic liaison with Keyes left her pregnant with a daughter.

ASTER
"Catherine Halsey"

- Déjà On February 23, 2525 Halsey got a chance to have a long chat with John during which, to her surprise, she learned he was convinced the Spartans were to enter a new, more severe phase of training. Writing in her journal, she expressed her desire to share everything she could concerning the upcoming augmentations and wondered if any of them would refuse were she to offer them the choice of whether or not to proceed. Déjà had continued to update and run Catherine's revision of Elias Carver's model weekly by this point, and the results were only growing worse. This data made Halsey more resolute in her conviction that nobody involved had any choice but to forge the Spartans. Five days later, she gave birth to her daughter, Miranda. Near the end of February Halsey and the rest of her team sent their prototype suit of Mjolnir armor to Naval Special Weapons for evaluation.

As the day for the Spartans' surgeries drew near, Catherine eventually summoned Soren to speak with her in her office. The fourteen-year-old boy could tell that she was overworked and not getting enough sleep and even after she instructed him to sit she continued to scan through electronic files while whispering silently to herself. When she was finally ready to talk she asked Soren if he could remember the day they'd met on Dwarka and inquired as to his thoughts on the choice he'd made to join the Spartan program. He confirmed he was glad he chose the way he did, but Halsey shared that she had wondered whether it was right to lay such a burden on him at that age. Brushing the issue aside, she then let him now she'd asked for him then because she wished to give him another choice. She went on to explain the next step she had planned for he and the others: their physical augmentation. Halsey did not shy away from telling Soren the risks the necessary operations and therapies posed. When he asked why she had singled out him alone to give the choice of moving forward in this way to, she claimed that he was essentially to function as the control group of the experiment but still could not say why he specifically had been chosen for this. After she had secured his consent the boy left and Halsey asked Déjà if she'd been listening and what her thoughts were on the matter. The A.I. pointed out that Halsey had misrepresented what a control group actually is and that she hadn't quite told him everything about the upcoming ordeal. In addition, she said she could tell that Halsey's purported justification for the reveal was spurious and it seemed to her more likely that the doctor was only trying to ease her conscience. Halsey could not disagree, dismissing Déjà to be alone with her thoughts.

On March 9, the day before Project: ASTER was to be initiated, Halsey and the seventy-five teens could be found aboard Medical Facility Endurance in orbit over Reach. At 1130 hours the doctor was pacing in the observation room while she waited for for Déjà to decrypt a transmission from Vice Admiral Ysionris Jeromi. Catherine had previously proposed a hypothetical series of experiments involving bonobos to he and his team in order to obfuscate her true intentions for the protocols. To her dismay, the latest from Jeromi informed her that further analysis had failed to yield viable alternatives that might mitigate any of the risks inherent to her "hypothetical" experimentation. As soon as she was finished with the file he'd sent Halsey erased all traces of it and even sent Déjà to track its pathways all the way back to the to destroy his related notes and files. Given the news, Halsey seriously considered delaying the procedures and only put such thoughts aside due to Déjà pointing out that ONI would only find someone else to move the project forward if she started dragging her feet. Chief Mendez met the doctor in the hall outside the observation room and at her insistence followed her up the stairs to the hospital's pre-op wing. Upon seeing John in Room 117 she forced a smile and asked how he was feeling. John, thinking of the procedures as just another exercise, asked her if she could just this once tell him how to win. To that, she could only lean in close and tell him he needed to survive. Later that day Halsey divided the various augmentations into two groups according to their risks of failure or long-term side effects not adequately studied. Those that she considered to have unacceptable fail rates were to be tabled for then in order that further research could be done. Already she had begun to quietly arrange long-term studies on the gradual application of cyclo-synthetic neural transmission gene sequences during subjects' early lives and noted in her journal that research on the injection of viral microphages at neuron surfaces was pending. Halsey closed that day's journal wishing there was a god to pray to. As she was being readied for the procedures within Lab 7, Halsey visited the girl Yasmine and assisted a nurse in strapping her down. She attempted to reassure the girl that she would barely feel anything apart from slight pain that accompanies injections. While visiting Serin-019 before the procedures commenced the girl asked her how different she would feel when she woke up. Halsey was honest with her and let her know just how painful and persistent the side effects of the surgical enhancements would be if she survived—possibly lasting months or even years.

During the augmentation process Soren-066 was returned to the Spartan compound on Reach, where he then woke during the procedures and violently attacked medical staff. The young teen was stopped in a hall beyond the operating room he'd escaped from by CPO Mendez and a line of five or six marines. Out of breath from running, Catherine arrived on the scene just after Soren was hit numerous times with tranquilizer darts, pleading for him not to be hurt. Soren's last words before collapsing were used to ask Doctor Halsey what she had done to him. When Project: ASTER's procedures were completed, only thirty-three of the teenagers had ostensibly been augmented successfully. As far as these thirty-three were made aware, twelve others had been injured during the operations so severely that they would be unfit to be Spartans and the remaining thirty had died. The truth, conversely, was that those pronounced dead were in fact cryogenically preserved in the hopes that they could be revived later, and some of those that were crippled would go on to be rehabilitated and join the ranks of Spartan-IIs in the future.

Aftermath
"Was the need commensurate with the risk? We had to move quickly. The onset of puberty was the gating factor—that and the pressure from a growing insurgent threat. Must move forward. Find solutions. I'm sure the vice admiral's ONI trauma experts are reporting that I'm 'not letting go'. They can all go to hell."

- Halsey relays her thoughts and feelings in her journal following the many failures produced by the augmentation processes. By March 30 Halsey had joined the forty-five who had unambiguously survived aboard the, an heading for the Lambda Serpentis system. She had told John that some of the side effects he was experiencing which were causing him to be clumsy were actually good, since they were evidence of stronger muscles and faster reflexes. The next day she reflected in her journal on how her training as a scientist had conditioned her to view failure, struggling to reconcile the scientist's duty to regard all data as useful with the malformations and excruciating deaths visited on her Spartans. She allowed herself to question whether the need to push forward with ASTER had been commensurate with the threat the Insurrection posed. Halsey guessed that Stanforth's trauma experts would be reporting she was having difficulty "letting go", but she committed to moving forward and finding solutions for those who might still be saved. To this end the doctor had ordered that no candidate autopsies be performed. The bodies of washouts were cryogenically preserved, pending further study. Her hope was that some, if not all, of them could someday be revived. Closed casket funeral services at midnight the day before, she noted, would help the survivors attain psychological closure. She was uncertain how they might react in the future to the resurrection of their comrades but was certain ONI would find a use for them regardless. Those now afflicted by serious malformations, after all, had already been offered various "desk" duties within ONI. Serin-019 was among those Halsey told the others had died, though in truth she had only been terribly crippled and would be delivered to Earth for ONI to take custody of her. Catherine had assigned Déjà to investigate as-yet theoretical therapies for the washouts' resuscitation. She was particularly interested in the potential of the complete restructuring of bones and attendant nanografts, something she thought could prove especially useful for Kirk-018 and René-081. Even so, she decided she would allow the washouts to adjust to their new jobs for several months before suggesting some of these experimental procedures. By then, she figured, they would be willing to try anything to recover, as she couldn't imagine them to survive the vigorous indoctrination and training they had to only become passive spectators. The Atlas was patrolling the Lambda Serpentis system by April 22 and it was shortly after 0430 hours on this date that ONI engineered a hostile encounter between John and four orbital drop shock troopers in order to test his capability. Halsey observed the encounter in the ship's gym remotely, cooly remarking upon the boy's steady heart rate and pulse. John had broken the nose of one ODST when their sergeant intervened to propose that they settle their dispute in a nearby boxing ring. Mendez, watching along with Halsey, questioned the wisdom of what was happening but his hesitation was rebuffed by her. She pointed out that John was only doing what they'd built him to do. As the chief left her to make his way to the gym she simply renewed her interest, wondering what effect the augmentations might have had on his adrenaline. Not long after the Spartans on the Atlas returned to Reach five of them elected to quit the program and escape. Daisy-023, Ralph-103, Joseph-122, Oscar-129, and one other boy managed to get ahold of M6C magnums and hold at least seven individuals at gunpoint within a hallway, one of whom was Doctor Halsey. Daisy declared that they were leaving and could not be prevented from doing so. Daisy eventually was able to secure passage offworld and traveled to Sargasso, where she had been born. Unbeknownst to her, she was followed by Halsey. Catherine caught up to the teenage girl outside her old home, having been flown there in an AV-14 Hornet. As Daisy was first laying eyes upon her own clone, Halsey chose to speak to her from the Hornet through a radio in her ear. She explained the rationale behind the production of the clones in 2517 and urged her to return home to Reach. In the end, Daisy, Ralph, and Joseph were brought back into the fold; Oscar and the other boy committed suicide.

It was not until early June that Soren, counted among the washouts, awoke from a comatose state. Repeatedly during the three months in which he was out it had been thought he would die and at one point the decision was even made to disconnect life support, but after four minutes of flatlining his heart had then begun to beat anew. Halsey paid him a visit in the brig where he was being held almost immediately after he woke up, flanked by an armed guard of two marines. She greeted the boy before looking him over and dismissing her guards, who were resistant at first but gave in when she claimed rank by virtue of being within a science facility. Halsey apologized for the restraints limiting Soren's movement and assured him she had argued against them but had been overruled. Following this she sat down on the floor within his reach and he joined her shortly thereafter. When Soren let her know he wasn't feeling quite right she said the analgesic medications he was on were likely to blame and that others were dealing with varying degrees of pain just like he was. She explained that his body had reacted poorly to the administered muscular enhancement injections and thyroid implant, ultimately resulting in intense musculoskeletal pain that she believed would diminish over time but likely never go away completely. She did note to him, however, that he had become stronger than anyone had imagined the Spartans could be. When Soren asked how many of his brothers and sisters remained, she told him almost half had died. At that she apologized before leaving without a further word a moment later. By June 27 planning was underway for experimental surgery, therapies, and rehabilitations for more than eighty percent of the washouts. Halsey noted in her journal that several of these protocols could take years to complete and that A.I.s were predicting a survival rate of no greater than fifty percent, but she also believed that her Spartans would exceed expectations just as they always had. The day which followed was the doctor's last official one at Camp Hathcock. FLEETCOM had offered her a corner suite in the Olympic Tower in New Alexandria, but she had turned it down in favor of CASTLE Base, which was to serve as her new home from then on. Her funding was shunted to higher profile projects and her access to classified materials was severely restricted around this time. Each morning she became forced to pass through a dozen security checkpoints and submit to a barrage of retina, voice, fingerprint, and brainwave identification scans after descending into the base's labyrinth to reach her office. On June 30, 2525 Halsey was shocked to learn Naval Special Weapons had approved limited-run production of Mjolnir. She had expected at least another three months to elapse before she got such news. She made a note to herself in her journal so she would be reminded to follow up with her source regarding what had prompted NAVSPECWEP's prompt decision. One file in the official response Catherine received also contained a partial routing code which she was able to backtrack through NAVSPECWEP's firewalls. Doing this revealed a volunteer had been put into the prototype suit against an emphatic warning she had made about the armor's reactive circuitry being fine-tuned for augmented Spartans. The man who had volunteered suffered the breaking of several bones and the snapping of his spine in three places. Halsey, in her journal, pledged that day she would find out who was responsible for ignoring the specific safety measures she'd put in place to prevent something like this and for failing to enable all the monitors.

Time of testing
"Catherine Halsey"

- Franklin Mendez By September 1, 2525, Halsey had begun applying the new liquid metal crystal version of polymerized lithium niobocene. Its amorphous nature allowed it to be "poured" into a capillary system where microelectric fields could direct crystallization geometries. The material scaled fractally to double a Mjolnir wearer's strength, but Halsey believed she could exceed even this were she to ignore what she considered archaic safety regulations. In her journal she defended such a move by arguing that nobody had anticipated this technology when such regulations were put in place. Over the next ten days, Halsey spent some time on another trip that took her away from the Epsilon Eridani system. At one some point during this period Déjà brought to her attention a transmission from the planet Harvest in the Epsilon Indi system which the A.I. had been attempting to decode. Halsey had Déjà show her what little she had and also asked for a feed from the planet, but nothing of the sort was accessible. The doctor urged her to keep trying.

On September 11 Halsey had just returned to Reach. Chief Mendez was surprised to find her reclining in his chair when he opened the door to his office at 0930 hours that day, as he had been under the impression she would be gone for yet another week. She told him their situation had changed and asked to know where the Spartans were, since she had failed to locate them in their barracks or on any of the ranges. The Chief told her they'd been forced to find them other facilities recently and offered to show her what he meant. He then escorted her to his personal M12 Warthog and drove them both off of the base and onto winding mountain roads leading to a series of titanium mines. They passed a manned guardhouse before descending steeply underground through a tunnel which wound downward in a spiral. Mendez revealed that dozens of old Mark I exoskeletons had been stored down there and were now being used by the Spartans' trainers, three of whom had been accidentally killed on the first day back from Lambda Serpentis. The vehicle came to a stop within a large cavern, Mendez helping her out and leading her to a small prefabricated one-room building. At the push of a button by Mendez the room climbed a track along the wall until they were twenty meters off the floor. When floodlights snapped on a game of Capture the Flag was illuminated in a large section of the cavern; trainers in armor defending against Spartans. Halsey could not see any of her Spartans at first, but she soon identified a glimpse of motion as Kelly. The trainers were armed with 30mm miniguns loaded with stun rounds, but when one almost landed shots on John at point-plank range the doctor briefly panicked, for even stun rounds could kill at that distance. Once the flag had been stolen without any sign of the Spartans remaining Halsey asked to see what she'd just witnessed again, sure that it had been recorded. Unfortunately, the teens had already found the cameras and disabled them. The Spartans' newfound impressive abilities made Catherine conclude there must be unexplained synergistic effects brought on by the combined modifications. Once she made sure to inquire about any physiological or mental instabilities, she then asked Mendez whether he thought they were ready to kill others in real combat. He believed they were, so she let him know something unexpected had happened and that the Admiralty wanted them deployed as a first test. Mendez rightly guessed that the rumors he'd heard about heavy action at Harvest had something to do with this, so it was left to Halsey to inform him that the colony there had been wiped out. She proceeded to bring the observation room back to the floor and told him she wanted the Spartans ready to muster at 0400 hours the next day for a briefing aboard the, a which would take them on a mission ONI had been saving for the right crew at the right time. Tomorrow, she said, they would see if all the pain they'd put the Spartans through had been worth it. At 0605 hours the next day both Halsey and Mendez met the Spartans in the Pioneer's briefing room while en route to the Eridanus system. The doctor walked to a the podium and greeted them with good news: Command had decided to finally put their abilities to the test. The doctor explained that intelligence suggested an insurgent base had been established in the Eridanus system's asteroid belt by rebel elements who had escaped Operation: TREBUCHET years before. She further introduced them to the URF leader Robert Watts and announced that they were to infiltrate the base to capture him alive to be returned to UNSC-controlled space. Halsey then stepped aside so Chief Mendez could go over the details of the operation. When finished, he and Halsey left the room.

MJOLNIR
At some point following October 7, Doctor Halsey officially requested access to the data archive of an RQ-XII Drone originally deployed from the and recovered by the  at Harvest. Her request was granted with a warning that all queries would be logged in accordance with protocol. Halsey searched the archive using the keywords "AO.AI", "Mack" "rampancy" and "lifespan limits". The data captured in the log was formed of messages from the agricultural operations A.I. Mack intended for Sif, the planet's shipping operations A.I. The garbled messages demonstrated the A.I.'s descent into rampancy over a period of eight months. The Pioneer did not return to Reach until November 2 of 2525. At 0600 hours that day Halsey met the Spartans within the same secure briefing amphitheater where she'd introduced herself to them years earlier. They were also joined by Chief Mendez and Admiral Stanforth, who informed the teenagers that Harvest had been annihilated some time ago by what appeared to be alien attackers. The adversary responsible called itself "the Covenant" and had broadcast a message announcing that the destruction of humanity was the will of its gods. Stanforth told them that the UNSC had been ordered to full alert and that the largest fleet action in human history was being mobilized in response. And because of the UNSC-wide mobilization, the Spartans' training was to be accelerated to its final phase: Project: MJOLNIR. Halsey spoke briefly with Mendez and Stanforth following the meeting's conclusion before leaving with the admiral. By November 6, Halsey and the Spartans had departed Reach aboard the. The next three weeks were spent handling a variety of minor missions under her guidance on their way out to Chi Ceti IV on the edge of UNSC-controlled space. All of the Spartans worked together to put down multiple insurrectionist factions on Jericho VII and also to remove a black market bazaar near the Roosevelt military base.

It was at 1750 hours on November 27 that the Commonwealth finally emerged in real space in the Chi Ceti system. Halsey then immediately summoned John to the bridge, where she waited with the ship's Captain Wallace. The doctor called his attention to a screen displaying deconvoluted radar signals, including one which was repeatedly appearing and might represent another ship eighty million kilometers away.

Alpha Corvi II
"Catherine Halsey"

- Linda-058 On December 3, 2525, Halsey met with Blue Team within the ready room assigned to it on the Persian Gate while still en route to the Alpha Corvi system. She stated that seeing as they had had the past two days to get used to the idea, in their gut, that a Spartan fall in battle, it was now time to put their grief aside. She told them that though they should honor that emotion, it would only be a hindrance to the upcoming fight they were sure to be sorely needed for. Lieutenant Commander Yao entered then and took it upon herself to brief the team on the Covenant's present attack on Alpha Corvi II. She explained the role Blue Team was to play in the defense of Jamshid, the planet's largest city, but then left the remainder of the briefing to Halsey. Once Yao had left, Halsey encouraged the four teens by letting them know the Mjolnir armor had performed better than had been hoped for during the engagement at Chi Ceti IV and that much of that had been due to their operating skills. And though she could say that UNSC Command was pleased, she reminded them that the Spartan-II project still wasn't trusted and that there was especially concern that their presence could negatively influence marine battle readiness. Luckily for them, she offered, Alpha Corvi II just provided another chance for them to prove themselves. Once an A.I. aboard the Persian Gate named Nora provided Doctor Halsey with updated medical scans for each member of Blue Team, she confirmed with John that he was well enough to fight. She also asked whether he would tell her if he wasn't to which he simply said that she'd find out if that ever happened. Taking her leave, she instructed the Spartans to suit up as they'd be exiting slipspace within thirty minutes. She later rejoined them after they'd arrived at the colony to see them off. During the Spartans' insertion via D77-TC Pelican, she remained in contact with them and let them know that while their first priorities were the neutralization of the enemy and the defense of Black Reef, a mining settlement they'd been redirected to after the Persian Gate was contacted by the ground, she also wanted them to try to find out why the aliens were targeting it specifically. She asked that they pay attention to the enemies' tactics, unit composition, weapons. As the craft drew near its landing zone, she closed by telling the four of them not to expect any help from marines on the ground, as Command didn't want the two groups interacting.

Over the course of the engagement that ensued on the surface, Blue Team followed a group of Covenant within the mines beneath Black Reef and contact was eventually lost with them. Yao was anxious to know what the Spartans were doing and was also overly concerned that they wouldn't be able to escape back to the surface, seeing as the mine entrance they'd entered through had been blown up. Halsey confidently asserted, however, that they were just following their orders and that their resourcefulness should not be underestimated. As the fighting wore on, both planetside and in orbit, the probabilities of both tactical victory and survival of the UNSC fleet began to decrease sharply. At the recommendation of Nora, Yao ordered a general evacuation order for all UNSC personnel on the surface and also ordered for Blue Team to be located so they could provide extraction coordinates. Halsey asked that one Pelican be set aside specifically for her Spartans but Yao just told her to ask again once they reestablished comm contact. Blue Team was able to exit the mine by moving underwater toward the shore of the island Black Reef sat on. Yao, worried, inquired as to whether Mjolnir was designed for submarine operations, to which Halsey admitted it wasn't but surmised that her Spartans must be improvising. The Pelican Tango 807 collected the team and ferried it back to the Persian Gate and Halsey met the four of them as they exited. John was incensed that members of the United Rebel Front who had aided them against the Covenant were being left behind to die, but Halsey tried to make him understand that every fight will always involve some collateral damage and that the most important thing was that he got his team out. She then pushed to find out if he had been able to ascertain what the Covenant had been looking for in the mines. The Persian Gate jumped from the system a short time later, leaving Alpha Corvi II to the aliens.

SILENT STORM
"Preston Cole"

- Catherine Halsey On February 24, 2526, Halsey puzzled in her journal over the meaning of the term "Covenant", specifically wondering whether it referred to an agreement between the various species or to a bond believed to be shared between them and a higher power they worshiped. She believed that finding the answer was necessary in order to better understand and exploit the aliens' psychology. In addition, she questioned how many races might belong to the collective that yet remained unknown to humans. On March 5 of 2526 twelve Spartans of Gold, Green, and Blue teams boarded a Covenant frigate above the uninhabited planet of Netherop in the Ephyra system. During the boarding action a sensor operator on a nearby prowler activated SQUADCOM and gave away the Spartans' presence. Following the ship's destruction Halsey shared a portion of their TEAMCOM download with the responsible crewman and his commanding officer, Lieutenant Commander Hector Nyeto, so that they could hear how the Spartans had reacted to being compromised. Nyeto later claimed that Halsey had yelled at the operator and by the time she let them both go he was so sick about the mistake that he was volunteering to find Daisy-023 to introduce himself. Unknown to Halsey then, the activation had actually been intentional, as Nyeto and those loyal to him wanted the Spartans killed on behalf of the Insurrection.

By March 7 she was aboard the  within a deep space transitional zone of the Dynizi system. At shortly after 0840 hours, in a compartment located within the Flag Deck's secure conference suite, Halsey met with vice admirals Stanforth and Preston Cole, who commanded the Everest. The three of them had met to plan a means by which the Navy could feasibly fight back against the Covenant. Catherine had also concluded that Staff Sergeant Avery Johnson of the Marine Corps would also be a valuable asset to have at their disposal, so he soon joined the meeting as well. When Johnson entered the room he found Halsey forcefully arguing the necessity of another attempt at capturing a Covenant ship, such a mission having been recently tried and failed by twelve of her Spartans. She argued that it was of paramount importance that they gathered as much of the aliens' technology as they could, since she believed the only way to beat the aggressors was through reverse-engineering it. She also was of the opinion that the best strategy at that moment would involve massing fleets at strong points, leaving most colonies undefended while she and others within ONI's Materials Group worked over the next year or two to deliver the ability to fight back. Halsey implored Stanforth to make FLEETCOM understand this, but ultimately backed down and admitted that she had been making assumptions about the aliens' strategy that were unjustified with what little they knew about them. In the face of this admission, however, she stressed once more the importance of capturing an enemy vessel. As far as she was concerned the recent try had not been a total failure thanks to the capture of a few Type-26 Banshees and the destruction of the ship. It was with reluctance that Halsey finally agreed such an acquisition couldn't be the prime objective of the upcoming operation—it would need to be secondary to blunting the enemy's advance against their worlds. When the conversation eventually turned to why Avery had been handpicked to assist with this op Halsey reminded him of his experience fighting the Covenant just a little over a year prior and his history as an ORION Project volunteer. She had hoped his Special operations background and creativity exhibited at Harvest would lend themselves to the creation of new infiltration tactics for each ship they targeted. Additionally, the doctor pointed out that his experiences with ORION made him able to relate to the Spartans, introducing the sergeant to what they were and how they had been trained. Stanforth described Johnson's role to be a sort of "big brother" to the Spartans during the mission, with Halsey making sure he understood he'd never be able to replace their squad leader.

The next day Halsey joined Johnson and both admirals aboard the  in deep space within the Polona sector. At 0558 hours she led John-117 into the ship's tactical planning center for a meeting with the aforementioned men, Captain Halima Ascot, and Colonel Marmon Crowther, commander of the 21st Space Assault Battalion of orbital drop shock troopers. Halsey took the first open chair around the TPC's conference table and before long was being introduced by Cole as leading their effort to analyze and reverse-engineer alien tech. The mission those present had gathered to discuss, Operation: SILENT STORM, was being conceived of as a desperate mission with the stated aim of boarding as many Covenant capital ships as possible in order to detonate small-yield tactical nuclear weapons within their hulls. When Colonel Crowther balked at these aims, Halsey suggested that John be put in command of his troopers. She had previously broached this suggestion with Cole, who rebuffed it for a second time, stating that John was not ready to lead something of that scale. Halsey began to argue in kind but was cut of by John himself, who agreed with the admiral. This earned him a scowl from the doctor but she did not press the point any further after that. Ascot, who would be leading an all prowler force dubbed Task Force Yama, let Halsey know she would be confined to the Vanishing Point for her own safety for the operation's duration. Halsey began to protest this but relented when Cole threatened to ship her back to Reach should she try to leave the ship. Following this meeting Catherine gave Johnson access to the Spartans' TEAMCOM to facilitate his role as liaison between they and the ODSTs and also made it clear to John that he should give the sergeant's advice a lot of weight—especially when navigating the vagaries of protocol inherent in their attachment to the 21st.

Seoba and Etalan
Task Force Yama reached the Kolaqoa system and the planet Biko's third moon, Seoba, on March 18. Seeing as Biko lay right in the path of the enemy's current invasion route, it had been picked as the perfect spot to lie in wait for the Covenant. The task force was soon met with resistance from insurrectionists working in concert with the Biko Independence Army during its insertion of Spartans and ODSTs to an abandoned ice quarry on the moon. The following day, Halsey was present at 0816 hours for an impromptu after action debriefing held around a folding conference table within Assembly Chamber 4L430 in one of the quarry's abandoned habitats. She was seated between Captain Ascot and Colonel Crowther; also present were Captain Nelly Hamm, Sergeant Johnson, Lieutenant Commander Nyeto, and John-117, who the debriefing principally concerned. During Ghost Flight's insertion Alpha Company had taken on an inordinate amount of casualties, and the unorthodox manner in which John had partially led that drop was being questioned. Halsey pushed back against the very notion that anything had gone wrong with the drop, seeing as the dockyards had been captured in less than an hour. She was surprised to hear, however, that it appeared the enemy had been expecting them. Regardless, Captain Hamm claimed that while still in the John had dumped a wounded trooper in her lap for the sole purpose of keeping her preoccupied so he could do things as he saw fit. Halsey was quick to defend him, but John confirmed the captain's accusation. Nyeto also spoke in John's defense, but in doing so let on that he may be younger than the age currently being claimed for him. As the conversation shifted to this matter Halsey protested its relevance and told Captain Ascot the true ages of the Spartans were compartmentalized and classified Top Secret Level One. She also assured her, however, that their training and judgment was unparalleled. Despite her efforts at further concealment the lieutenant commander explained how he'd realized the Spartans must all be around fifteen years old then. He claimed to have a friend who had trained with a bunch of eight-year-olds on Reach seven years prior. Halsey responded by leaning forward so her upper body shielded Nyeto from Ascot demanded how he could know those eight-year-olds were her Spartans, and once again it was John's inability to convincingly lie that had given away the truth. The doctor was finally forced to resort to saying the Spartans' ages could not leave the room, but Colonel Crowther was quick to threaten her with incarceration for violations of the Uniform Code of Military Justice. Catherine asked the man if he really thought his superiors would let the Judge Advocate General hear about this before emphatically declaring that despite their youth, the Spartans were not children. Ascot and Crowther determined that they were to be sidelined for the remaining duration of SILENT STORM—something Halsey strongly urged them to reconsider.

After a long silence the debriefing returned once more to the matter of Alpha Company's casualties.

Naraka
"Catherine Halsey"

- Déjà At 0626 hours on March 31 Halsey was in the middle of studying the captured starholo. The Vanishing Point had relocated to the deep space Rendezvous Point Sierra-Yama within the Polona sector. With a little luck and lots of hard work, the doctor was hoping she could trace the Covenant's slipspace routes back to the invasion fleet's primary supply depot. By this point she had already identified more than twenty transit nodes which opened near what she believed were Etalan, Harvest, and Netherop, and four of these were near other nodes which led out of human-controlled space. Five days had passed before a new supply convoy had arrived at Etalan following the previous one's destruction, so Halsey surmised its point of origin had to be somewhere between one-half and three-quarters of that distance at the aliens' slipspace travel rates. She asked Déjà how certain the A.I. was about the size of the logistics base that the Covenant fleet must require, but she only reconfirmed what had already been calculated previously. Catherine was flummoxed, since there simply weren't any habitable worlds within four days of the likely transit nodes. Déjà deigned to point out there was one within five days of Transit Node Bhadra, however. Implicit in this observation was the idea that the Covenant must have instantaneous interstellar communications.

By this point Catherine had discovered a data miner had been slipped into her quarantined lab by Hector Nyeto or one of his operatives. It was causing Déjà to make erratic pauses while speaking and had already stolen her most closely guarded data, but Halsey was tolerating it because she didn't want Nyeto to know she had noticed it yet. Nonetheless, the doctor agreed with what Déjà was suggesting and determined they ought to proceed to Transit Node Bhadra. With that settled the A.I. notified her Colonel Crowther had finally arrived. Catherine deactivated the starholo and ran her fingers through her hair briefly before meeting the colonel at her door. She'd sent him a message asking him to meet her there as soon as their rendezvous was complete for a "personal matter", and now that he'd shown up she feigned delight at his presence and took his arm to draw him toward her personal quarters. She confessed to being unable to wait any longer to be alone with him and made sure to press her flank against his as she pulled him through the other door into her little galley area. Once they were both sealed within, however, she released him and said she only needed to talk someplace safe. She let him know she swept for eavesdropping devices within her quarters three times daily and had found nothing so far. She offered the men water and apologized for leading him along before stating they needed to discuss how they were going to handle Nyeto before he tried to kill her Spartans again.

Through researching Nyeto's records Catherine learned he had spent a lot of time with the infamous Robert Watts early in his career. She and Crowther told John that though they would leave him alone as the task force's commander for the time being, they planned on using him to mislead the enemy. Despite the improved slipspace efficiencies afforded by Doctor Halsey's understanding of the alien starholo, it still took the task force to arrive at Transit Node Bhadra, more than two hundred light-years past humanity's farthest known exploration. It was Halsey who had named the transit node, and she likewise chose to call the sector of Covenant space they were invading by 0458 hours on April 14 Muruga, both Hindu names meant to honor Captain Ascot. Their target planet and its moon she called Naraka and Rudara, respectively. By April 15 Halsey had viewed reconnaissance videos which showed Naraka was encircled by a ring of orbital support stations connected by a transit tube enclosed within what she called a trussing armature. In her estimation, if Sierra Force could destroy at least ten facilities and their connecting armature there was a ninety-two percent chance the ring's orbit would grow unstable and fall into a rapid, irreversible decay. If only nine were destroyed, she predicted the odds of that happening dropped to seventy-one percent, and if only seven, forty-nine percent. Ultimately, Operation: SILENT STORM scored a giant win for the UNSC when the orbital ring was successfully brought down thanks to her Spartans and Crowther's ODSTs, but the colonel lost his life and Hector Hyeto fled from the rest of the task force during the mission.

Total War
"Vice admiral ordered us to repurpose everything to quell the Covenant threat, and to refocus our research on wartime strategy. I couldn't agree more (it was, after all, my idea)."

- Catherine takes note of the UNSC's shift in priorities within her journal. Thirteen days later, on April 28, Halsey noted in her journal that Vice Admiral Stanforth had ordered she and others to repurpose everything to quell the Covenant threat and to refocus their research on wartime strategy. She couldn't have agreed more, seeing as doing just that had been her idea to begin with, though she did sarcastically ask if the Insurrection would adhere to the policy as well. By this time Halsey had learned ONI Section Two had sent emissaries to leaders in the Outer Colonies to warn them of the Covenant and to make them offers of peace. Footage from the glassing of Bliss generally convinced these leaders of the magnitude of the threat they all faced. In her journal Halsey wondered if the aliens might have solved their civil war—not a trade she would have made, but perhaps a small victory nonetheless. On May 3 Catherine reviewed in her journal the distinctions between so-called ""smart" (volitional) and "dumb" artificial intelligences and why the "smart" ones are susceptible to rampancy. She noted that she had devised new matrix strata with random-access optical couplers that could theoretically extend a volitional A.I.'s lifespan, though by exactly how much she was unsure. In simulations a randomly coupled matrix also boosted correlation effects such as accurate guessing. As volitional A.I.s were needed then more than ever to counter the Covenant threat, Vice Admiral Stanforth had authorized a test run of Halsey's protocols. After leaving herself a reminder to investigate the polarization of topological soliton pulse transfers she concluded that day's writing by questioning whether she was playing the role of God or mother in breeding life from non-life, or perhaps simply being beguiled by sentimental anthropomorphism.

Near the start of May John-117 told Halsey he suspected the Covenant had created a special Spartan-hunting unit based on encounters he'd had with well-trained warriors in distinctive armor during Operation: SILENT STORM. In response the doctor immediately distributed a list of precautions and procedures she wanted implemented whenever her Spartans were in a combat theater.

Before Blue Team was dispatched to Netherop to investigate a downed Covenant frigate, Halsey sent the Master Chief a message admitting that it was most likely a trap but she still needed him to go on the mission due to what they could learn from it. On June 12, 2526, Halsey took part in a Court of Inquiry for the Master Chief following the events of the Battle of Netherop. Halsey disagreed with Admiral Cole's initial assessment of the operation, pointing out that it was clearly a trap likely meant to capture a Spartan, though they were able to get their hands on a personal shield generator of a type they'd never even seen before. When the Castoffs requested to go to Gao, Halsey was so shocked she went so far as to slap a hand to her brow. However, after the two gave their word to the Master Chief, Admiral Cole agreed to the deal and the Court was dismissed.

At some point after her Spartans begin racking up successes, Halsey's meager budget mushroomed overnight and she was once again offered a corner office in Olympic Tower. She declined the offer for the second time, however, as she enjoyed the fact that someone had to spend a half a day getting through various security barriers to meet with her. Her original banishment, as she saw it, had become a bureaucratic weapon for her. At some point in late 2526 Halsey caught up to Soren-066 and Chief Mendez as the two were conversing while walking in a hall on Reach. The Chief had been explaining why active service was no longer an option for the boy and Halsey jumped in to back him up, emphasizing that Mendez had to do what was best for the program and the others trained in it. The doctor promised he would still be able to serve in a non-combat role and regretfully apologized when he asked why she was no longer allowing him to make choices. At some point more than six months after she told Soren that hard truth, the teen helped a technician at the compound steal a GA-TL1 Longsword. They did not get far, however, and Soren was forced to crash-land the fighter. Chief Mendez paid a visit to Halsey in her office after visiting the site and looking through the wreckage and let her know that no signs of Soren were present. Only his accomplice's body was found. Halsey vowed to look into how this technician—Partch—had gotten into the position he'd held, given that his background showed all the earmarks of an insurrectionist. Mendez recommended she run another background check on everyone higher up in security. Halsey then stopped him before he could start smoking right there in her office and asked if there was anything else to discuss. The chief went on to relay further details about parts torn free of the fighter which had landed elsewhere, but despite there being no sign of Soren among them either, he thought it unlikely the boy could have survived. When Halsey asked how Randall-037—who had unsuccessfully tried to stop Partch and Soren—was doing, Franklin claimed he was fine. After being told that ground troops had been dispatched to comb the woods for a body the doctor told him to pull them back and have Soren filed as missing in action. She wagered that he was still alive, but based on her experience with him that meant he would only ever be found if he wanted to be. Mendez started to protest but she reached across her desk to touch his arm and urged him to let him go, arguing he was not a threat to them. Mendez pointed out that he had been a trained and augmented as Spartan and now had rebel sympathies, but Halsey only replied that he was no traitor; just a lost soul looking for direction.

On December 11, 2529 Halsey recorded that she'd had limited success using the illegal entry protocols to access a nonessential subroutine in a recovered Kig-Yar point defense gauntlet. The details of her success were published as part of the work of another researcher, Doctor Hardy, who had been studying the gauntlet. Unfortunately, it was demonstrated that using the protocols in this way accelerated the terminal cross-linkage effect of her A.I.s. Jerrod, a micro-A.I. in her employ, believed he had a solution to this predicament. Halsey was skeptical, but agreed to proceed according to his advice with caution. On April 9, 2530 the doctor made note that Chief Mendez was incommunicado and that nobody seemed to know his whereabouts. She decided to continue monitoring this development, as she felt something must be wrong to have pulled him off her radar like this. At some point prior to 2531 Halsey took on at least one student, Ellen Anders. The two had a mutual hate for one another according to Anders' later recollection. On July 12, 2531, Halsey noted that Section Three had begun to use a new-generation A.I. of its own to conduct surveillance and protect its assets. She had noticed that her network codes, keystroke style, and even her voiceprint all triggered its counter-intrusion measures. Though she could devote time to clandestinely pursuing this to sate her curiosity, she knew she'd be detected. As such, she was left to just wonder what ONI was hiding from her. It was by this date as well that Jacob Keyes had agreed to take in Miranda, which her mother figured would be for the best. By the end of October that year, Halsey had recently purchased upgrades for her Spartans' microfusion plants. On January 18, 2532, Halsey recorded her thoughts upon learning that several of the chemical precursors used in Project: ASTER had been routed to Section Three laboratories on Algolis, Meridian, and Ariel. She was enraged at the possibility that the Spartan-II program might be being replicated without her guidance and couldn't fathom what the implications of that might be. She made sure to track all those who had "washed out" during the augmentation procedures in 2525 but as far as she could tell, none of them had been repurposed. Those that had been rehabilitated were on track to be recommitted soon and whatever was going on, she reasoned, must not involve them. The doctor noted the peril at which she was maneuvering outside of Stanforth's protection and the terms of her charter. She figured her enemies must think the threat of being charged with espionage or with the violation of any particular military regulation would keep her from pursuing leads in order to preserve her position, but she judged that to be an underestimation of her.

Research Advancements
"The illegal entry protocols ("PIE") used for hacking insurgent networks have deciphered a tiny portion of the aforementioned Jackal shield system. Just goes to show that no scrap of genius goes wasted! I've had Jerrod dedicated all his runtime to further avenues of inquiry—perhaps they can crack more. Already have enough data to run a simulated test for a Mjolnir energy shield system. Moreover, if we can understand their operating systems, we hope to gain insights into their language (their motivations?). That might prove ultimately of more value than any weapon we could devise."

- Catherine Halsey responds to a major breakthrough with enthusiasm. By November 10, 2533 Catherine had developed a new theoretical architecture which involved three artificial intelligences arranged in parallel. She did this in order to palliate the preemptive severing of neural linkages and ultimate self-termination which characterized third-generation A.I.s, hoping that by leaving all choices to be decided by majority vote the frenzied cutting behavior would be avoided. In her design, ties which arose when one of the A.I.s abstained would resolve at random. Though Halsey knew this could slow overall processing, she was confident such a theoretical trio would deconvolute and divide algorithms to compensate for the loss, and in doing so perhaps even accelerate processing. Consensus-based decision-making would also be applied to linkage creation in order to forge a superior initial neural linkage. Unfortunately, Halsey could not know how the theoretical trio's overall lifespan might be increased under such circumstances because simulations could not create in vivo A.I.s. Writing in her journal on that November 2533 date, the doctor wondered what other characteristics could emerge from an A.I. triumvirate. She only had three at her disposal at the time and as such had moved them to the Sanderson private hospital on Beta Hydri VI as a precautionary measure. In addition, she kept all knowledge of this from other A.I.s she interacted with. A note from Halsey to herself on November 10 served as a reminder to follow up on reports of nested A.I. personalities that were present on Harvest before it was lost. A little over a year later, on December 25, 2534, Halsey recorded her thoughts concerning an interaction she'd had with a colleague, Doctor Forester, in which she had illustrated increasingly complex sketches for him with the intent to visualize conceptions of space with increasing hyperdimensions. She had been attempting to show him that it was the increased density of cross-linkages which caused wave function collapse, not the creation of the links themselves. If space were not a limiting factor the number of available connections would increase at a geometric rate. Though Halsey considered classic string theory dubious, she thought the eleven (or more) dimensions it predicted could theoretically be accessed by anyone outside the quantum fields which enveloped ships traveling through slipspace. She was disappointed Doctor Forester apparently failed to grasp that slipspace was open to exploitation within four-dimensional spacetime. Writing in her journal following their discussion, Catherine pondered whether remote construction with any exotic matter in the raw slipstream was possible, for at that moment the technology required for that remained at best theoretical. She figured decades or even centuries may pass before the necessary tech and its practical applications arrived. She closed that day's entry wondering whether people were ready for the potentially near-limitless cross-connections and faster-than-light processing speeds A.I.s in slipspace could possess.

By January 7, 2535 Catherine had reminded Admiral Stanforth at a budget meeting that continuous upgrades were integral to the Mjolnir system if they expected it to shatter technological barriers as a pioneering effort (something she'd thought they'd both agreed upon). To her dismay, she was told this would no longer be possible since the military required a simplified generational system to categorize, prioritize, and serialize the project for fiscal year budgets. Halsey agreed to cooperate with this stipulation but determined she would by no means wait on breakthroughs just to fit accountants' calendars. She understood she was working with systematic impediments, and on January 7 put down in her journal her preliminary modularization of upgrades for MJOLNIR with the expectation that the entries could and likely would be modified or reconfigured as time, budget, and other limitations cropped up. She also considered that a reevaluation may be in order as new technologies and limitations manifested—especially if the Covenant noninstrumentality problem was cracked. In particular she had to wonder whether energy shielding would be possible for the Mark V armor, seeing as there was no evidence early prototypes had been useful so far. She made a note to herself that day to reach out to Doctor Yin-Finto to find out whether she had taken a research position on Titus and to ask if nanofabrication of Mjolnir components would be possible. Two days later Halsey was scheduled to meet Chief Petty Officer J. G. Jimes at 1800 hours aboard the, an encounter which at least partly concerned Beniese coffee. By November 22, 2535 Halsey had received what were being referred to as the top secret "Hencheck Findings", named for a Lieutenant Hencheck who had procured a cache of Covenant weapons and miscellaneous artifacts during a skirmish in the Indra asteroid cluster. Much about the enemy's weaponry astonished her and made her ask questions about their construction. No traces of production designs could be found within the weapons' architecture, and much of the technology seemed heavily polarized yet uniformly purposed, leading Halsey to think it might have originally been developed for some other purpose or purposes. Regardless, the immediate conclusion she drew from the findings was that humanity still lacked the ability to replicate the Covenant's technology. By January 10, 2536 Catherine had gotten Leonard Gillespie, a student of Doctor Forester's, transferred to Reach so she could watch him closely. This was due to his recent discovery that a portion of the inner apparatus of a particular Kig-Yar point defense gauntlet in the UNSC's possession had not been sealed. In addition to the gauntlet already being clearly much older than any others encountered, initial testing confirmed that with a massive power input its capacity for creating and sustaining energy shielding over a very small surface area could be replicated. The implications such a technological leap could have for MJOLNIR made Halsey determined to get her hands on the artifact. By May 25, 2537 Halsey and Kalmiya had deciphered what they thought could be a root diction to a meta-Covenant language system. Elements of this system had been observed manifesting in the (potentially misnamed) "operating system" the aliens used. Curiously, employing their symbolic logic appeared to counter the accelerated cross-linkage effects in A.I.s. Though her progress in understanding this was moving slowly, it was advancing steadily nonetheless, as she wrote in her journal. She also made a note reminding herself to investigate the possibility that what she was working with might represent dimensional slices of a more complex topology.

On March 18, 2543 Doctor Halsey finally found her journal after having lost it for some time and was faced with the decision as to whether or not she should transcribe notes from a new journal she'd been using into it or even shuffle papers and rebind them. In any case, seeing as she couldn't always keep it on her, she recommitted to keeping it secure from low-tech prying. Writing in it on this date, she even mused over the possibility of implanting a small strip of nanocene explosive into the clasp and keying it to her DNA before admitting she'd likely employ a more refined method. By March 21 an experiment which involved severing portions of an A.I. from its "living" matrix strata and then reintegrating data and connections had gone flawlessly. Halsey was profoundly relived by the results because no literal "cutting" of the material had been involved and the reintegration of the separated A.I. portion with the mother A.I. strata represented a real innovation. Optical repeaters were maintaining the boundary value wave functions at zero within the original A.I., but it was recommended that the separation lasted no more than eight days. Her experiment demonstrated that excised parts of an A.I. could learn and evolve autonomously and still then be reabsorbed into the parent matrix. As this was only an experimental process, however, Catherine decided to designate it as "non-sanctioned" until she more carefully weighed its long-term ramifications. By May 20, 2544 Halsey had moved her laboratory a few times since she'd last written in her journal and had almost thought it lost in the process. On that date it was discovered that the illegal entry protocols used for hacking insurgent networks had deciphered a tiny portion of the point defense gauntlet acquired over eight years prior. In the journal Halsey then added them to a list of Section Three counter-insurgency strategies which had proved to be notable exceptions insofar as they had actually been effective against the Covenant. Following this she directed Jerrod to dedicate all his runtime to further avenues of inquiry. Even then, however, she was sure there was already enough data to run a simulated test for a Mjolnir energy shield system based on the Jackal's gauntlet. It was her hope that greater understanding might lead to further insights into the Covenant's language and motivations. With so much going on and with so much to say, Halsey resolved to write more going forward.

Capture and rescue


"John-117"

- Catherine Halsey At some point during that same year the Inner Colony of Miridem came under assault by the Covenant while Doctor Halsey was present there. The colony was declared lost after over two weeks of fighting. During the civilian egress which followed Halsey and her bodyguard, Sheila-065, managed to evacuate aboard the craft BL-9400493, which was also the only one among several to not be shot down. Halsey was cryogenically frozen in preparation for the jump to slipspace, but their ship was intercepted at the edge of the planetary system and disabled by the enemy before it could slip away. Sheila gave her life protecting the doctor, which indicated the value of Halsey as a prisoner. She alone was spared and brought aboard the Third Fleet of Glorious Consequence's flagship, Resplendent Fervor. The day after losing Miridem, Vice Admiral Berlin M. Tursk sent a message from the to Lucius Jiron, captain of the  PRO-49776, in which he told the captain he would be receiving Spartans shortly and that he ought to prep his OF92 booster frames for a quick and dirty extraction of the doctor. Continuous pinging of her beacon by the Swiftsure indicated she was still alive and still cryogenically frozen. By the time the Spartans had been received the Third Fleet of Glorious Consequence was being prevented from initiating slipspace jumps by the powerful magnetic field of the star Ascon.

When Halsey was finally awakened from cryostasis she found herself facing none other than John-117, who had infiltrated Resplendent Fervor to rescue her. He asked if she'd slept well, at which she joked she evidently had, no thanks to the Covenant's driving. She teased that perhaps John's luck had rubbed off on her, and when he swore he'd always be there for her she stopped him short, saying that he shouldn't make promises he can't keep. Regardless, she was thankful despite also pointing out that it wouldn't be a proper rescue until they made it off the enemy carrier. They hastened from the chamber where her cryotube had resided, rushing down a long hall until they were met on the other side of the door by Thel 'Lodamee, the same Elite who had killed Sheila. John opened fire with his MA5, but the Elite's Type-1 energy sword was able to block the bullets. Surprisingly, 'Lodamee tossed the Spartan a second sword. Halsey watched as the two began to duel. At that moment the fleet became clear of Ascon's magnetic interference, however, and the shipmaster decided to disconnect the portion of the ship John, Halsey, and Thel were in before jumping to slipspace. Halsey realized something had changed when an alarm began to ring throughout the ship and cautioned John to wait during a brief respite in the duel. The major was suddenly caught up in a gravity field and pulled back to the foremost section, leaving Halsey and John trapped in Delta Section. Knowing that what remained would be scuttled soon, the two ran to a bay at Halsey's suggestion and boarded a Ren, blasting away just before the ship was wracked by explosions. They were accosted by a group of Kai-pattern Seraphs left behind by the fleet, which led to Halsey frustratingly asking if there was anything John could to do save them. Thankfully, the combined efforts of PRO-49776 and another Seraph commandeered by both Kelly-087 and Fred-104 kept them from being killed. Once aboard the prowler, John let Halsey know that Solomon-069 and Arthur-079 had been killed during the effort to retrieve her. Feeling somewhat rattled by his fight with 'Lodamee, he stated his determination to be stronger, to which Halsey agreed, saying they all needed to be. Though the war was in its twentieth year something told her there was much more ahead of them.

A.I. and Slipspace
"Incredible. [10141-026-SRB4695], with minimal influence from this Assembly, has attempted to build an abstract fractal structure within Shaw-Fujikawa Space. Although her first attempt was a failure, success could finally remove our dependence on biological systems altogether! If our Minds could somehow achieve freedom of expansion within eleven-dimensional space, immortality might be within reach."

- An artificial intelligence of the Assembly Minority theorizes on what Doctor Halsey's experiment could mean for the future of A.I.s. On September 3, 2547 Halsey noted in her journal that following her inquiries regarding the assumptions of the mu-tensor calculation in Doctor Canterfeld's latest paper in I.C. Physical Review, Canterfeld had finally asked her for help with the areas of his research that were continuing to elude him. He had responded to her inquiring by declaring that Shaw-Fujikawa space is "literally slippery" and then proceeding to explain why his assumptions were valid. Halsey was glad to help him, as she could see that his theories about slipstream anomalies could lead to a number of innovations, such as those which would result in faster ships and more accurate jumps. Catherine was convinced that humanity needed such advantages to counter the Covenant's superior technologies, noting at the same time that she was starting to doubt the idea that their enemy possessed superior intelligence. In addition to the benefits aiding Canterfeld could bring to the war effort, Halsey also wondered if his contacts and support in the United Nations could effect a transfer for her. She chronicled her thoughts on the matter, questioning whether being permitted to devote herself to "pure" research would cause her to lose sight of how many were depending on her. She concluded, that with so many trails of her studies converging at that moment she could not leave, even if she did view her work as a sort of burden, purgatory, or dharma, in spite of Admiral Stanforth's assertions regarding her "destiny". In the end, she wrote, it may be all that remains to her, having given up a normal life, her family, her Spartans, and even the "illuminated entities" which she said she had come to understand. She was certain she wouldn't be able to make Canterfeld understand why she had to refuse him, so she decided to just send him a note the following day. That next day, the battle-worn frigate arrived in dry dock at Reach, with its slipspace drive slated for disposal at 1615 hours. This was fortuitous to Doctor Halsey, who saw that drive as the last disparate component of several she had secretly acquired for an experimental procedure. One other of these was a probe from the planet's early warning slipstream space observatory. In her journal Halsey mused over whether she possessed the mettle to steal the drive and conduct the experiment and couldn't help but think of fate and luck, despite her best efforts to reject entertaining such notions. She intended to build an abstract fractal structure within slipspace with an A.I. seed inside it.

Once she secured access to the drive, she first needed to breach the external, internal, and vacuum coupling shields to the singularity, for which she bemoaned that the UNSC couldn't make a proper pan-torque screwdriver. She would have preferred to use hardened remotes for the breaching, but they would have been detected by networks which would also record their activities. As a result, a peculiar synesthetic effect moved along her left arm up to her neck when she removed the shields. She also experienced a kind of "hiccup" in her awareness which she surmised was due to the delta time which could occur with abrupt transitions into slipspace. Next, she had to check the probe's telemetry systems after compensating for how close the experiment was to Reach's gravity well, and found they were within tolerable variances. Halsey then verified the data of the A.I. seed "H-7" to seven significant figures and initiated its transfer to the matrix strata within the probe. After starting the slipspace drive and verifying the data received, as well as activating the AI seed, she confirmed an initial growth state. Upon checking and triple verifying the probe's telemetry uplinks she then finally launched it on an aligned trajectory. Recording her observations in her journal, the doctor noticed again that time seemed to have mysteriously elapsed, as there was a step anomaly in the chronometer. This had not affected her or her wristwatch as far as she could tell, and she guessed that the anomaly might be a technical artifact or a subdimensional leakage from the probe as it was inserted. It was only then that she realized she had already made a note of this "hiccup" closer to the time it had occurred but could not recall doing so. Telemetry confirmed that A.I. growth was beyond exponential—appearing instantaneous. Halsey wondered variously if this might be explained by the temporal discrepancy, some hitherto undiscovered multidimensional effect on the A.I. cross-linkages, or even alternate physical laws harnessed by the unshielded slipstream. Much of the incoming data and consciousness cross-checks soon became garbled and voice communication from the A.I. was attenuated. Logic/mathematics Scriber tests confirmed full functionality, but Halsey was disturbed by the conclusions, insights, and even what she considered nonsense being generated. The A.I.'s comments on humanity and the Covenant defied reason from her perspective, but she made sure they were all recorded, including those concerning other intellects in the "mist". Halsey wondered in her journal whether the "mist" referred to slipspace and also commented on another four-hour "step" in time that might also be explained by lost consciousness. She additionally noted that the data-transfer buffer had been blown—capable of one hundred and forty-eight hours at its maximum rate. As soon as she made this observation Halsey set about replacing it. After the telemetry became fully operational she noticed glaring gaps throughout the record. And although the voice communications continued to grow increasingly nonsensical, all rampancy indicators were negative.

Before telemetry was lost, a communication burst provided Halsey with mathematical equations which she could not make sense of, leading her to wonder what she was missing. As contact with the probe had terminated on her end as well, she sealed the drive's three shielding layers and considered the experiment over. She noted that she had no desire to repeat it and hoped that the A.I., now irretrievable, was "dead". Catherine then ported the collected data to Jerrod for further analysis, filed and encrypted essential records under her personal code, and triggered a viral worm and bit grinder with the intent to destroy all other digital witnesses. She closed her documentation of the experiment in her journal with an ambiguous remark concerning one last thing she had to do. Despite her attempt being somewhat of a failure, the A.I. collective known as the Assembly took great interest in it. During an emergency convention soon after its conclusion an A.I. of the Minority predicted that a successful repeat of what she'd done could lead to immortality for its kind. Ultimately, it was determined that further experimentation of the like would need to be actively encouraged. The day after, Halsey speculated in her journal that artificial intelligences might in fact be "more alive" than their human creators. By this point she had begun to check the KIA lists on the UNSC subnet daily and had come to the realization that most of the friends from her youth were dead. The closest friends left to her then were the A.I.s that surrounded her.

Cortana
"I'm delighted with Cortana. She has assumed a rather simple human appearance (much to my relief; so many of the ONI A.I.s adopt such theatrical personas). Her face resembles mine when I was fresh out of college—so wonderful and naïve, and eager to learn. It's almost like looking in a mirror."

- Doctor Halsey's written thoughts on Cortana's chosen avatar.

On March 24, 2549 Halsey made a note in which which she reminded herself to soon finish field tests experimenting with direct human-to-A.I. neural linkages. She believed such a linkage would serve as the final necessary upgrade incumbent to Mjolnir's Mark V platform, as well as the last gift she had to offer her Spartans. An exceptional A.I. was needed to assist them with Operation: RED FLAG, a last-ditch effort to capture a Covenant hierarch. Though Halsey felt she had no other choice but to proceed in this direction, she did worry that in combining Spartans and A.I.s it felt as if she was losing both somehow. On May 21 Doctor Halsey cloned herself twenty times, thereby breaking civilian laws, UNSC military regulations, and (far from the first time) the UN Colonial Mortal Dictata. She believed her third-generation "smart" A.I.s required better material to use as A.I. matrix seeds than that offered by the new candidates she was being presented with, which she saw as lacking both physical and intellectual agility. To create the clones she used the same equipment that had been used years prior to make the flash clones needed for SPARTAN-II. This time, however, she enhanced their neural physiology at the expense of other biological functions. She made sure the equipment was hidden and secure from all save herself, though the cloning had been arranged by ONI at her insistence. As such, there were a few others who also knew about it, but she was confident that these particular individuals could be trusted to keep the information secret. The clones developed at a breakneck pace, for on the same day she had first begun the process they were already exhibiting familiar anatomy and curling into fetal positions, malformed as they were. In her journal Halsey noted how disquieting this was to watch, grieving for them and wishing she could allay their suffering somehow. A little over two months later, on July 29, Halsey successfully excised all of the clones' brains and transferred them to cryogenic units. All remaining organic substrates were flash-incinerated. She planned at this time to complete her personality transfer seven weeks later and was still uncertain whether or not she should tell them they were clones of her. It was at some point in 2549 that UNSC High Command finally made the Spartan-II project public in order to boost morale. Halsey protested this at first, but ironically the publicity proved convenient because the attention on the Spartans' heroics distracted from questions about their origins.

On September 9 Halsey reflected on the recent loss of Arcadia, as she had come into possession of a near-complete record of the attack taken from from an orbiting weather satellite. In her journal she expressed the agony viewing the destruction caused her, noting that she knew some would smirk at the thought of her shrinking from it. As the entire world's surface was mercilessly glassed rather than only portions, she wondered variously whether the planet posed a threat to them, had religious or political value, or was simply designated for sending a message to humanity. By September 14 Halsey had hacked into the redacted portions of other Arcadia reports. On November 7, 2549, within the confines of CASTLE Base, Catherine Halsey transferred and authenticated the H-1 specimen's neural pattern to A.I. matrix strata, completing the initiation sequence within normal quantum flux tolerances. The newly-made intelligence called herself "Cortana". Halsey placed a viral termination code in her kernel but then could not tell whether she remained unaware of it or had already isolated and bypassed it. Halsey also programmed her with ONI's best insurgency software and every ONI computer insurgency routine, as well as the determination to use those code-cracking skills. She first designed and tested these routines on Kalmiya. Only once the process had been debugged and streamlined did she incorporate them into Cortana. Afterward, the brass in Section Three instructed Halsey to destroy any prototype routines—an order which she promptly ignored.

The first words spoken by Cortana were those Halsey's mother had said to her after beating her daughter in chess decades prior. Halsey recognized in the choice of phrase a confluence of morality, mortality, and metaphor. In her journal she recorded her delight and relief upon seeing that Cortana chose a simple human appearance for her avatar. She also mentioned that specimens H-2, H-3, and H-4 would remain in cryogenic suspension, as she had other plans for them which were dependent on evolving technology. On February 11, 2550 Catherine recorded a research excerpt in which she took note of certain metrics which implied the viral termination code in Cortana's matrix would have been unlikely to be successful and that the typical seven-year life cycle estimate may not apply to her. She'd determined that the unique circumstances of her creation had triggered what Halsey could only refer to as a "recessive variant" in the A.I. seed, but the doctor was at a loss concerning this rogue element's origin.

Xenoarchaeology and other investigations
"It's interesting that you should bring up Halsey. I know she'll likely be involved in anything we do with RED FLAG, but she's been on the end of a very long leash for a while now and my patience is wearing thin. Since she stumbled onto that artifact in Visegrád, she's been rummaging around in the wrong file cabinets—mine, and more specifically: the data we collected from Onyx. At this point, I'd be shocked if she hadn't pieced together what we believe the planet actually is, but I'm keeping her in Viery with half a dozen eyes watching her every move."

- Margaret Parangosky, Commander-in-Chief of ONI, bristles at Halsey's voracious unsanctioned investigations in a message to Vice Admiral Stanforth. On April 6, 2550 Halsey reflected on her history as a mother, regretting her inability to dissuade Miranda from joining the military and her personally having arranged for Miranda's assignment to the. Miranda had recently been one of only two people to make it off the Hilbert alive after she'd given the order to sacrifice it to take out a Covenant destroyer threatening three UNSC corvettes. As such, Keyes was presented with a Silver Star, the new commander's insignia, and command of the. She'd invited her mother to the ceremony after years of not speaking to her, but Halsey hadn't gone. In her journal she noted that while she could explain her refusal by pointing to the fact that Jacob would be there or the half-dozen projects she was neck-deep in the final stages of, she knew the real reason but didn't want to put it to paper. By June 3 of that year Professor Laszlo Sorvad had begun an archaeological study of the debris field near the village of Visegrád. With his help and without Halsey's authority, Section Two "leaked" that the site may contain a precolonization meteorite metallurgical and astrophysical scientific value. Halsey wondered why ONI didn't just say the location was a dumpsite for toxins and other radiologicals, but she noted that this leak gave her more insight into how Section Two thought—bolstering her knowledge of what to watch for in the future. She also began urging Sorvad to migrate the local population elsewhere but he did not heed her advice. It was around this time that Catherine then began to investigate files under the purview of CINCONI Margaret Parangosky without permission, especially those related to the planet Onyx in the Zeta Doradus system.

On November 23 the Official Secrets Act of 2550 was handed down, regulating the confidentiality and nondisclosure of any official messenger satchel. This additional measure struck Halsey as onerous and when she made inquiries regarding it to Stanforth he uncharacteristicallt deflected them. Catherine speculated in her journal that the new regulation might relate to Visegrád or some of the other rumored sites she'd been collecting data on, and while she did appreciate the need for secrecy she did not appreciate relevant data being kept from her. If indeed Visegrád was what she and others all thought it might be, she reasoned—and if there were other such sites, she wondered who could be positioning themselves to assay the larger picture. By February 20, 2551 the, the UNSC's largest medical facility, had departed Reach with a minimal escort. According to Halsey's own sources, it was accompanied by one ONI prowler in addition to a single destroyer. The Hopeful's A.I. was unusually tight-lipped when Kalmiya asked about its destination. This, plus the late addition of carcinogenic, mutagenic, and experimental chemicals to its manifest among other oddities, piqued the doctor's curiosity. She questioned in her journal why Stanforth would keep any clandestine Section Three medical experiments from her. To her irritation, inquiries she made on the matter were met with dead ends. The facility returned by February 27 and Vice Admiral Jeromi even invited Halsey to come aboard for a look at its new flash-clone organ facility. Once they were together he abruptly steered their conversation to tell her he knew she was "fishing" before she could even ask him anything related to the ship's short absence. He also claimed that he knew very little himself and couldn't tell her what he did know, under threat of death. By May of 2551 Catherine had begun to lead the excavation of an ancient alien vessel that lay buried beneath the area occupied by ONI's SWORD Base within the Babd Catha Ice Shelf in northern Eposz. Even so, her residence remained in Csongrád where she also continued to work with Doctor Sorvad on the Visegrád findings. On May 8 she was interviewed by analyst B.M. Parkes so that they might conduct a Hemley and Aubourn behavior analysis of her. Following the encounter Parkes recorded that she had not been terribly forthcoming and ultimately recommended assigning two to three handlers to her at both sites where she was working. Parkes also was confident she needed to be surveilled were she to be called off-planet, though her leaving Reach was something they and their colleagues strongly discouraged at that time. Two days later Halsey noted in her journal that a security issue which she blamed Sorvad for had caused her to begin keeping an eye on the inhabitants of Visegrád to ensure they didn't disappear thanks to Section Three. As an insurance policy, she told Stanforth that the people there for valuable for being the only ones with extended exposure to the site under study and as such may need to be studied. She met with Sorvad and he assured her that only certain members of the dig team and a few other "necessary parties" on her side had seen the items recovered from the site. He insisted on continuing the work with only a skeleton crew of his most trusted people and promised he would not tolerate "another Farragut Station", referring to the communications outpost. Halsey simply nodded but was displeased, wondering if Sorvad might actually know what had happened at Farragut Station in the past and the reason for its relocation. On May 15 Kalmiya caught a glimpse of several documents in Vice Admiral Stanforth's Beta-5 server which ranged from service order transfers of existing Spartan-IIs to highly-classified non-Spartan-II detachments. This had evidently been going on for years, and Halsey was incensed at having been kept in the dark about it. She resolved to not let this go, and as she had a list of names with one man she considered particularly obscene at the top, she planned to send Kalmiya back in as soon as she got a handle on the affairs with Sorvad.

On the morning of July 18, 2552 Catherine got to have a brief talk with Admiral Stanforth concerning the protracted battle that had just concluded at Sigma Octanus IV. This followed the normal reporting on it and by the end of the day she had also read through both the admiral's after-action report on the battle (given to her by him) and the press releases put out by Section Two. Halsey's interest was piqued when she saw Jacob Keyes' name and that of his ship, the, featured prominently in the reporting, so she found herself skipping ahead to confirm he had survived. In her journal she surmised that the maneuver being dubbed "the Keyes Loop" had been inspired by one of Admiral Cole's. A heavily redacted section of the after-action report that detailed, as far as Halsey could tell, a groundside mission involving Blue Team, was of the most pressing concern to her. She was unsure why Spartans would have been committed there, seeing as the planet was in all probability about to be glassed at the time, but even Kalmiya was unable to break the section's encryption. The Covenant's return to the planet following its first repulsion led her to conclude it must have wanted something on it, so she resolved to follow up on this line of inquiry. Two days later Stanforth pretended to have never given Halsey access to the files he had, which made her think others must have been listening in on their conversation. She had noticed a recent intensification of paranoia and security around her, after all. She was certain she was missing something in the reporting which he wanted her to see that should be glaringly obvious. The next day, July 21, Sorvad sent her a brief, cryptic message on his Theta channel. He claimed to have made a latchkey discovery that was too important to send through the data link he had with him, and since he could neither leave the field to deliver the information in person, he said he would send it through the Visegrád communication relay hub. After more than six hours since his message Halsey had still received nothing, but she decided against second-guessing him and elected to simply continue waiting.

NOBLE Team
"All the pieces are almost in place for Operation: RED FLAG. My curiosity, for once, must wait. If one small team of Covenant have infiltrated Reach, more will certainly follow. I must prepare for the worst-case scenario with WHITE GLOVE."

- Catherine, writing in her journal, puts aside her confusion concerning the Spartans of NOBLE Team in order to focus on more pressing matters. By July 23 contact with the relay hub had been lost. Given how the communications network had been hardened years back and the urgency and demand for secrecy in Sorvad's message, Halsey ruled out a rudimentary malfunction, believing the loss to be more than a coincidence. She made a short list of possible explanations for the blackout, none of which were pleasant. She figured either internal subterfuge within ONI, insurgent action, or even the Covenant could be to blame. Halsey made sure Stanforth, who had personnel addressing the situation, was aware of her vital staff in the area. She also shared her thoughts on the potential causes with him and was told she'd be notified when her people were located. The next day she caught a rumor that a special forces unit had been sent to investigate the hub but that it too had disappeared. Halsey learned that a second unit named "NOBLE Team" would be sent to investigate next, but she could not get anyone to give her any further details. Without even knowing what branch of the armed forces this NOBLE Team belonged to, she resolved in her journal to follow up on it as soon as possible. Meanwhile, the doctor was also receiving scattered data from the usual suspects concerning atmospheric anomalies along with radar and slipspace monitoring glitches. She compared these events to those recorded near Sigma Octanus IV but there were no matches. By 1126 hours on July 26 SWORD Base had come under attack by an and Covenant ground forces from the Fleet of Valiant Prudence. Following the attack's repulsion Halsey requested permission to debrief with NOBLE Team, the six members of which met her within the base's science wing thereafter. Upon seeing them her suspicions were confirmed: they were Spartans, as incomprehensible as that was to her at the time. Only one of them, Jorge-052, was one of hers. She had already begun conversing with the team's leader, Carter-A259, when Jorge and sixth member of the team finally joined them. She was in the midst of telling Carter to dispense with telling her about the recent engagement outside so she could hear about what happened in Visegrád when Jorge's entrance made her stop mid-sentence to greet him. She couldn't help but ask after the obvious modifications made to his armor before returning to the matter of the Visegrád relay, and Doctor Sorvad's death in particular. From Carter, Jorge, and Spartan-B312 she learned that a group of three Sangheili Zealots were likely responsible. The same had ambushed NOBLE Team when they'd arrived at the relay and the one which appeared to be their leader had gotten away from the Spartans. Halsey dismissed mention of Sorvad's daughter Sára in the interest in maintaining focus on the Elites and what their leader might have escaped with. She bristled at Carter's defense of his order not to pursue the last zealot, worried that it might have stolen valuable ONI excavation data. She let them know about the supposed latchkey discovery Sorvad had made and emphasized its importance, stressing that they should hope a data module found on his corpse contained what was so important. At this Kat-B320 handed the module over to the doctor through a transaction drawer, as a wall of glass separated her from NOBLE. She let the lieutenant commander know she'd been alerted when she'd tried to access its contents, threatening her with imprisonment for attempting to interfere with her work. This prompted Carter to rush to Kat's defense, citing the WINTER CONTINGENCY directive and the provision against civilian interference with a Spartan deployment to make the same threat to Halsey. Jorge lingered when the rest of the team began to leave, but Halsey told him they'd talked enough, allowing him to move on as well.

The data in Sorvad's last message to her had been compromised along with the entire relay hub, which had been effectively destroyed. Following her meeting with NOBLE Team Halsey noted in her journal that security measures were under way to preserve ONI's discoveries in the Viery Territory and to integrate and update Cortana with the latest intelligence from the Visegrád site. In addition, she was able to confirm definitely that the five with Jorge were indeed Spartans. She pulled Jorge's service record, but more was redacted than was usual even for a Spartan and she had trouble reconciling his recent activity with Naval Special Weapons. Seeing how he had seemed to grow in the past two decades, she made a note to herself to check pituitary and human growth hormone levels for enduring catalytic side effects. Halsey figured Jorge must be glad to be back on Reach, even given the circumstances, but was unsure whether or not she was only displacing or projecting her own feelings onto him. The others on NOBLE Team, of course, were a mystery to her, but seeing them did remind her once more that Mendez was missing. If time had permitted, she would have pursued this thread, but as all the pieces for Operation: RED FLAG were almost in place, she had to wait. The Covenant's presence on Reach was also enough to get her to begin preparing for Operation: WHITE GLOVE, which involved denying the enemy access to classified data.

By 0250 hours on August 1 Admiral Stanforth had provided Doctor Halsey with a list of assets required for RED FLAG. The mission would require between twenty and thirty Spartan-IIs, the Mark V Mjolnir software suite, an at least third-generation A.I. extensively programmed with Covenant system intrusion software, a refit or augmented, and a veteran naval commanding officer and crew. Half of these would be coming directly from Halsey, who had already chosen Cortana to serve as the mission's A.I. By August 3 Halsey and Cortana had both agreed she ought to be divided after long discussion. John and the other Spartans would need her expertise for RED FLAG, but she was also the best-suited to continue the study of symbols and operating systems from Professor Sorvad's site. In her journal Halsey pledged to copy most of Cortana's illegal entry protocol translation routines to an independent module so so the incarnate personality which remained could go with Blue Team and the rest. It was her hope that the severed data-mining portion could be reunited with the rest of her so she could be updated with Sorvad's data before the , RED FLAG's designated cruiser, departed. Halsey had chosen the Autumn for the operation because it could take an incredible beating without suffering functional compromise far longer than any other ships the Navy was currently fielding. By August 5 the independent module for Cortana had been successfully copied and all tests indicated separated routine functions were within normal parameters. Halsey noted in her journal that she hoped the remaining work left to Cortana wouldn't take much longer. She would need to leave as soon as she was ready, provided the Autumn was ready in time. Pledging to catch up on lost sleep later, she also made a reminder to herself to inquire with Cortana concerning how it felt to experience a multiplexed existence. At some point during the seventeen days following her meeting with NOBLE Team Catherine got to talk with Sára Sorvad about her father.

The artifact from Sigma Octanus IV
"From this supplemental data (which I couldn't copy and had to replicate from memory), I've hypothesized that the language/code, and hence the object, derives from predecessors of the Covenant, or from an unknown, more ancient alien culture. Deciphering the message on the object, however, eludes me for the moment. A more definitive analysis certainly requires the actual object and all relevant etiological data. I only shared this with my undisclosed ONI counterparts because I knew it would get back to Ackerson and "inform" him that he possesses an authentic alien artifact... One that he would need me to decipher. If so, I'll get everything he has."

- Catherine muses in her journal about how best to get more information on the Sigma Octanus IV findings. Battle Group Leviathan arrived at Reach from Sigma Octanus IV by August 12 so Halsey went on that date to see what had been done to her Mjolnir units. Replacement parts and possible upgrades were available, though she noted later that day that John-117 would need to be laser scanned for the new cybernetic implants she planned to give him. Once she was aboard his, the , Jacob Keyes came to see her as she had been both hoping and dreading. They were able to talk about Miranda, his recent "Keyes Loop", the Spartans (including their operational records, which she showed him), and what little she could prematurely disclose about Operation: RED FLAG. When they parted they wished one another the best on their respective missions and Keyes even kissed her on the cheek when she offered her hand to shake. By 1100 hours later that day she was present for Keyes' debriefing at Camp Hathcock, sitting on a panel which included herself, Admiral Stanforth, and Colonel James Ackerson. A little over ten minutes later she met John within one of the camp's military complexes—where he had just finished his own debriefing. She was sitting in an overstuffed chair when he returned to the debriefing lounge and her surprise presence caught him off guard. She asked how he was doing, making sure to not give any suggestion that all was not well on Reach at that moment, as it had been decided the Spartans should be kept in the dark for the time being so that RED FLAG might not be compromised. In the few minutes they had Halsey told him about the upcoming procedures planned for him and gave his head a cursory MRI. John asked after Mendez and all the others the two of them had called family and Halsey shared what little she knew without giving heed to protocols or security clearances. They then sat together quietly for a time. John was enthusiastic about the victory at Sigma Octanus, so she asked whether he would like another victory and perhaps the biggest they'd ever had at that. She left things with him there, however, and just offered that they'd be speaking soon. Turning to the military police attendant waiting at the lounge's entrance, she instructed him to open the doors so she could take her turn speaking to the committee of twelve naval officers, ONI and otherwise. Two days later Halsey was within SWORD Base again when it saw further bombardment from the Covenant. On August 23 the Spartans of NOBLE Team were informed they were to redeploy to SWORD Base for a torch-and-burn operation meant to keep Doctor Halsey's excavation data from falling into enemy hands. Unbeknownst to them, they were actually being recalled there so the doctor could use them as couriers for the fragment of Cortana which would need to be reunited with the rest of her before the Pillar of Autumn departed. At some point prior to their eventual arrival on August 30, while within her lab at Babd Catha Catherine reviewed what she knew about the team's members with Cortana, seeing as one of them would need to personally take possession of the fragment for delivery. Cortana elected to give that role to Spartan-B312, the team's newest member—a decision Halsey agreed with.

On August 24 Doctor Halsey attended a debriefing where she was shown a hologram of a piece of an object Blue Team had recovered from Sigma Octanus IV—though she was not made aware of its point of origin or even what branch or group within ONI had seen fit to share it with her. This, of course, hindered her from performing a complete analysis. Even so, it was plain to her that the artifact shared many iconic language and code forms with the alien ship buried beneath the area around SWORD Base. During the debriefing she candidly speculated that the artifact must come from a precursor to the current Covenant society or an entirely different alien civilization. Once the meeting concluded she was finally given permission to proceed with Project: MJOLNIR's final phase. In her journal later that day she puzzled over a possible connection between what she judged to be the more "standard" stylized glyphs on the Sigma Octanus IV artifact and took note of potential similarities between other glyphs and hypothesized navigational code found in Covenant transmissions. She ultimately could not draw a conclusion and even wondered if perhaps illegal entry protocol deconvolution was to blame. Halsey hypothesized at this time that the artifact's creators might represent predecessors of the Covenant, if not a culture far more ancient and removed. She was unable to copy the data the hologram displayed and instead had to commit it to memory. Deciphering the message on the object would require actually having it and all relevant etiological data, so she shared this with her undisclosed ONI counterparts knowing it would get back to Colonel Ackerson. She suspected it was in his possession and was sure she could get everything he had on it once he realized her help would be needed to decipher it. At 0915 hours the next morning Halsey was fourteen minutes and eighteen seconds late to her private office within the Omega Wing of CASTLE Base (which she had returned to several days earlier), as Déjà pointed out as soon as she entered. She told the A.I. she could blame security for that, as ONI's precautions were becoming increasingly ridiculous in her mind. As if to further solidify this view, when she sat down at her desk and tried to read the files pertaining to the prior day's debriefing she was prevented from doing so due to Déjà's presence. Though Halsey had fought hard to keep her free from the programming shackles ONI demanded, the doctor had to put the A.I. in standby mode before she could go any further. Looking over the files revealed that ONI didn't have any idea what in particular the Covenant had been doing on Sigma Octanus IV, to Halsey's consternation. When Cortana's avatar suddenly appeared and announced she too had read the report, Catherine halfheartedly scolded her for it before asking for her insight regarding the findings. Through the A.I. admitted there was insufficient data for a complete analysis, her conclusions generally matched those of her creator's.

RED FLAG
"Some of you already suspect this, but I shall state it anyway for emphasis. It is my opinion, and that of many others, that the war is not going well... despite our recent victories. What is not widely known is how badly it is going for us. ONI predicts that we have months, perhaps as much as a standard year, before the Covenant locates and destroys our remaining Inner Colonies... and then moves against Earth. Your mission will prevent this. This op is considered extremely high risk. There are unknown elements involved and we simply do not have the time to gather the required intelligence. I have persuaded FLEETCOM not to order you on this mission. Admiral Stanforth is asking for volunteers."

- Doctor Halsey lays bare the significance of what she is asking of her Spartans. While still speaking with Cortana that August 25 morning, Doctor Halsey let on that the final phase of MJOLNIR had been approved. Her creation asked when she would be leaving for RED FLAG, to which Halsey answered "Soon." There were still a few final modifications she needed to make to the Mark V systems which the Spartan-IIs would be using. Cortana inquired as to which of the Spartans would be carrying her so Halsey asked which one she would like to. Cortana immediately gravitated toward an image of John-117 but Halsey suggested she make a different choice, citing compatibility issues. At this Cortana swiftly developed a custom interface buffer that, once a new neural interface was installed, would allow her and John's neural patterns to synchronize within 0.081%. Catherine asked if she'd read his career service vitae and learned she was looking it over again right that moment. Both of them agreed that he was the bravest, the luckiest, and the best of the Spartans. The doctor asked if Cortana would be willing to sacrifice him in order to complete the mission, and once she'd assented to this, gave her permission to make him her carrier. Next, they discussed the Pillar of Autumn and determined refit operations needed to begin at once. Halsey had chosen Jacob Keyes to captain the cruiser so she forwarded Cortana his CSV so she could look it over as well. She was not immediately convinced the doctor had made the right choice, but Catherine pointed out his ability to keep a secret made him superbly qualified for the mission.

On August 27 Catherine Halsey and Jacob Keyes met with twenty-five of the remaining Spartan-IIs at 0800 hours underneath the Highland Mountains FLEETCOM complex to brief them on Operation: RED FLAG, as Admiral Stanforth had requested she do. There, in the same amphitheater in which they had been brought as children thirty-five years prior, she asked them to disregard normal protocol and ask any questions they may have during her presentation. She explained that RED FLAG was to be executed in four phases. First, a Covenant ship would be disabled. Then, the Spartans would board it and crack its navigation database. John interrupted her at this point to ask if mission specialist personnel would help with the Covenant computers and she assured him that would be addressed soon. The mission's third phase was to involve taking the captured ship to the Covenant "home world", and its fourth would see one of its leaders captured and brought to UNSC space. John spoke up again to question the wisdom of capturing a prophet rather than killing them, so Halsey addressed his concern by pointing out that killing one could trigger an escalation. Taking a hierarch prisoner might allow the UNSC to force a truce. Intelligence indicated that the three hierarchs at the apex of the Covenant spent their time either at the hegemony's mobile capital city High Charity or aboard either s or s. Capturing an assault carrier or supercarrier would be preferable because with luck one of the San'Shyuum High Prophets would be aboard and a trip to High Charity would be unnecessary. Hesitantly, the doctor shared that ONI was predicting only months remained before the enemy found Earth and moved against it. Even so, she had persuaded Stanforth and FLEETCOM to not order any of them to join this operation—all would be volunteers. Without a second through every Spartan present stood upon hearing this, and Catherine thanked them. As she stepped away from the podium she'd been addressing them from she said she and Keyes would meet with each of them individually within a few days to continue their briefing. Each of them needed to be given Mark V Mjolnir and shown how to get their "computer experts" on board the alien ship, after all.

In order to pull RED FLAG off, it had been determined that a colony of significant value to the Covenant and boasting many ancient alien artifacts would in all likelihood need to be sacrificed in order to draw the required Class-Five assets, possibly with hierarchs, in so that Spartans could infiltrate them. The best shot at taking over a large ship would come after a fleet believed it had secured a major victory. Given the current situation on Reach, ONI had decided to make the most of a losing fight and enact RED FLAG there. It was for this reason that not a one of the twenty-five Spartans was yet aware that Reach was already under seige, largely thanks to the efforts of Halsey and Stanforth, who had been making sure to keep them well away from the primary target sites. Later that day Halsey reflected on the meeting in her journal, being hopeful for the Spartans' ability to secure a temporary peace that would but humanity time. She expressed her thrill at seeing so many of them together once again and mourned for those who had died years past, truly surprised at how emotional the gathering had made her. She was confident they were ready, but still resolved to appear strong for their benefit. She noted that the Mark V armor they'd soon be gifted with was one small operation and a few tests from completion. With only a few days left before she expected to learn whether all her discoveries and sacrifices had been worth it, she closed that day's entry wishing there was time to properly analyze her feelings and reminding herself that four small tasks still remained to complete. By this point she had begun splitting Cortana into separate programs in order to juggle the Visegrád findings with RED FLAG. Cortana, for her part, was continually updating her separated portion and would keep doing so until the last possible moment. Catherine was excited to see how she and John took to one another. She had assured Stanforth that Cortana remained fully functional even with a large portion of her processes excised. By 0600 hours on August 29 Catherine and others were waiting for John at the "Prep and Recovery" area of Military Reservation 01478-B. She entered a prefabricated structure there just as the Spartan was having the back of his neck swabbed to tell him it would only take a moment to upgrade a few components of of his neural interface. She asked him to lie back and remain still, and once the procedure began she assisted to some degree with fusion of the new components. She took a brief moment to note in her journal that the surgery had gone flawlessly and that his recovery would be well within regenerative parameters. At that moment tests to evaluate the integration of the Mark V Mjolnir and A.I. were planned to commence in around three hours. Once John was finished receiving his upgrades she asked him to follow her to a field command dome on the reservation's range. After presenting her credentials to the military police standing guard and submitting to identification scans she led John in to introduce him to the Mark V armor. Halsey asked him to put it on, averting her eyes as he did so with assistance from two technicians. She explained hundreds of minor technical improvements had been made compared to the Mark IV and that she would send the specifications to him later. The two most significant additions, she added, were the armor's energy shield system and a weave of memory-processor super-conductor which allowed it to carry an A.I. She taught him how to adjust the shielding and assured him he'd get to see how much punishment it could take soon (she herself had suggested live rounds for the upcoming testing) before finally introducing him to Cortana. The A.I. had been provided with the very best Covenant language translation software, Halsey said, so that she could infiltrate their computer and communications systems. Halsey promised Cortana would not be able to control his armor when John asked, though she would improve his reaction time. It was her explanation that, like him, Cortana would do whatever was necessary to complete their mission that relieved the Master Chief. He knelt so Halsey could insert her memory processor matrix into his helmet and then watched with great interest as the Spartan and A.I. met one another for the first time.

Though the tests were not scheduled to commence for a little less than three hours, Halsey decided then that they should begin straightaway. She told John and Cortana that some within ONI did not believe they were ready for RED FLAG and so a simulated combat scenario had been arranged. Whispering, she even suggested that some would prefer they failed the testing and may have made arrangements to make sure they did. John confidently asserted that he would not fail and Halsey responded by saying she knew he wouldn't. She ordered him to count to ten after she left the dome and then to make his way to reservation's obstacle course and ring the bell at its far end. After telling him to be careful and granting him permission to neutralize any threats in pursuit of his objective, she and the techs took their leave. The test which followed nearly killed John, as an AV-19 SkyHawk made multiple passes over the obstacle course and fired upon him with its 50mm cannons and a Scorpion missile. The moment the Spartan rang the bell Doctor Halsey's voice broke in over his COM channel, calling for Colonel Ackerson to call off his men and congratulating John on his accomplishment. She also told him to stay put so she could send a recovery team to collect him. Turning to Ackerson, who had been observing the course from the same room as her, Halsey pointedly asked if he was now satisfied with Project: MJOLNIR. He gruffly dismissed John's success as due to luck, which Halsey countered by saying Spartans make their own luck as she left.

Fall of Reach
"Catherine Halsey"

- Carter-A259 Doctor Halsey entered her SWORD Base office shortly after 0400 hours on August 30. At checkpoint three she was briefly delayed by a reset of the verification system, which required her to speak that day's code phrase and undergo a retina scan twice in order to proceed. Unbeknownst to her, these interruptions were caused by Cortana, who the doctor had wrongly thought she could keep occupied at that moment with checking the systems on the Pillar of Autumn. The doctor had sent an official protest against Ackerson to the, backed by Admiral Stanforth, but was still tracking his possible involvement with the missile incident from the previous day. She had also suggested he may have suffered a mental breakdown and advised he be taken into custody, pending evaluation. She had received reports of either he or his agents investigating the area around CASTLE Base's xenoarcheological site in addition to scattered ones which alleged increased Covenant activity in the vicinity as well. In her journal, she surmised that Ackerson was unlikely to find anything of interest there and cynically expressed hope that he might cross paths with the enemy. Before NOBLE Team's arrival Catherine had a number of tasks which she was determined to make sure were accomplished. She prepared Kalmiya, finalized and compiled a firmware update for the part of Cortana which remained with her and upgraded that package for transfer, sent a note to Miranda by way of the, enabled a network purge, and prepared all backups for an Umbra slipspace shot to Earth. The reports Halsey had seen made her confident Reach was surely lost, but having completed what she'd needed to she felt prepared in her lab as she waited for the Spartans. NOBLE Team had arrived outside SWORD Base by 2616 hours on August 29 (local time). Shortly before the team reached Doctor Halsey Jerrod let her know SWORD Base's perimeter had been compromised, despite assertions to the contrary being made by the base's own A.I. In case something were to happen to her, Halsey recorded a purge code and its override failsafe password in her journal. Upon entering SWORD Base the surviving members of NOBLE were led at Cortana's direction to a tram which would carry them one-and-a-half kilometers east and around two thousand feet underground, where Halsey's laboratory was. Once the four of them had climbed aboard she appeared on a monitor and apologized to Carter-A259 for the unusual security measures which had kept them from fully knowing what to expect at the base. He started to remark on the fact that she'd been listed as having died, but the doctor dismissed the casualty reports relating to her as false before he could finish and said she only wished the same could be said for the rest of his team, both Jorge-052 and Kat-B320 having been killed within the last few weeks. She offered what might have been a small comfort in sharing that the data module delivered by Kat had unlocked the secrets of her nearby excavation. She then explained that their orders to destroy all sensitive material at the base had been overridden, as they had always been nothing more than a pretext to bring them to her so that they could take possession of this vital data and deliver it elsewhere. Carter argued that he would need to confirm this change with Colonel Holland, but Halsey assured him he would be briefed and said that they belonged to ONI now. As soon as NOBLE team first laid eyes on the massive ship Halsey explained that it was alien but not Covenant in origin and that thanks to Kat's data its decryption was nearly complete. Carter urged her to have it ready when they reached her but the doctor insisted the decryption was still underway and, should they allow any of it to be left buried there, they'd be burying mankind's best chance for survival. Covenant attackers soon began encroaching on her lab and she implored the Spartans to defend it in order to buy her all they time they could. As NOBLE Team fought off successive waves of enemies Halsey occasionally updated them on the progress of the data's final decryption, and as soon as it was complete she let them know she was opening the laboratory's door. Within, she explained to them that Cortana was the important data's custodian and had been the one to choose them as her couriers. Emile-A239 found the notion strange, and at that Halsey made it known that this A.I. was special. She then told them they were to carry her to the ship-breaking yards in Asźod, where they would find the  waiting to spirit her away. Carter indicated his understanding of the mission, but Halsey made sure he understood the importance of it, claiming that humanity's annihilation was all but certain unless they could gleam some way to stop the Covenant from the information the ancient ship had provided. With that, she removed the armored matrix holding Cortana from where it had been plugged in and handed it to SPARTAN-B312. She said that Cortana had chosen him specifically to carry her and made him confirm he'd taken secure possession of the matrix before finally letting go.

Halsey accompanied the Spartans to a cliffside hangar on the surface where two D77-TC Pelicans sat on a platform overlooking the vast area beneath which the ancient ship lay buried. Carter informed her that she was to be escorted back to CASTLE Base by Jun-A266. She briefly protested this, claiming that she required no escort, but Carter ignored her and coolly reminded Jun that nothing could be allowed to fall into enemy hands. The two groups then parted ways, Jun and Halsey heading for CASTLE Base and the other three for Asźod and the Pillar of Autumn. The Autumn had been about to initiate a slipspace jump away from the doomed planet at 0647 hours, but Halsey managed to get a transmission to Captain Keyes just in time so that he redirected the vessel to land at the ship-breaking yards where NOBLE Team was expecting the cruiser. She assured Jacob that he could count on Spartan-B312, Cortana's chosen courier, to deliver her fragment and its data to him.

Refuge under CASTLE Base
After arriving at ONI's CASTLE Base, Halsey volunteered to remain behind to ensure that all UNSC technology was secured from possible capture. She initiated Operation: WHITE GLOVE, destroying several AIs to keep the Covenant from accessing UNSC databases. With expectations that the Covenant would glass the planet and knowledge that even if they didn't, she would be facing an entire army of Covenant ground forces, it was clearly an offer of sacrifice.

However, this changed when Spartans Fred-104, Kelly-087, Vinh-030, Isaac-039, and Will-043 appeared at the entrance to CASTLE Base. With the firepower to potentially fight their way out, the possibility of surviving CASTLE Base's destruction became available. She gave emergency aid to the Spartans for their injuries, but a data anomaly detected by her accompanying AI, Cortana's "older sister" Kalmiya, attracted Dr. Halsey's attention.

The data had been accessed by Araqiel, an AI that worked for Colonel Ackerson, one of Dr. Halsey's main competitors within ONI. After tracing the data access, Dr. Halsey stumbled across the AI, left behind by its master. Alerted by Halsey's intrusion, Araqiel began to threaten her; he went as far as telling her he would pressurize the room, or fill it with narcozine gas if she did not cease. Instead, however, she typed in archaic line commands to access Araqiel's code directory and personal fail-safe, which she used to shut him down. With Araqiel terminated, Halsey proceeded to read through Ackerson's files and discovered a number of things including the location of Onyx, evidence of the SPARTAN-III program and a map of the mining tunnels CASTLE base had been built upon. The latter particularly interested the Doctor as the file had been classified at X-Ray level, making the file abnormally important for ancient maps. From this, she deduced that there was something of importance under the mountain.

Assuming that at the very least these maps provided a backdoor, she chose to move the Spartans out of the base and into the mineshafts. She then triggered CASTLE Base's self-destruct mechanism and activated the fail-safe on Kalmiya, erasing her to prevent her from falling into Covenant hands. With the Spartans in tow, they moved into the mineshafts.

Over the course of the next five days, the Spartans and Dr. Halsey searched the mines, looking for either an exit or what Dr. Halsey would dub "the most important discovery of the millennium". On the fifth day, Fred discovered an entrance into a large Forerunner structure, and within, a spacetime-warping crystal. However, the acquiring of the crystal sent a spike of neutrino radiation, attracting the attention of the Covenant. Their location was now threatened by Covenant forces.

Following a Covenant attack, Halsey and her team retreated into a hallway and sealed off the entrance, losing contact with Isaac and Vinh in the process. However, they quickly discovered that the hallway was a dead end. It is presumed that Halsey and the surviving Spartans remained stranded at this location for some time.

Escape from Reach


"I'm tired of sacrificing others for the 'greater good'. It never stops, Cortana... and we're running out of people to sacrifice."

- Halsey to Cortana before her departure to Onyx.

Days later, a group of Spartans, including John-117, rescued Dr. Halsey and the remaining Spartans and evacuated them from the Forerunner complex on Reach to the captured -Ascendant Justice. After escaping Reach, the group fled to the rebel base Eridanus Secundus in the Eridanus system's asteroid belt, where the Spartan's first mission had been in. During the voyage, she successfully resuscitated the clinically dead Linda-058.

Unbeknownst to the rest of the group, however, Halsey had her own agenda in mind. Examining Ackerson's files in CASTLE Base, she had concluded that the colonel had been conducting his own Spartan project which had been kept secret from her. Overcome by her guilt over the Spartan-II program and believing the war could not be won, she was determined to prevent any more Spartans from dying by taking all of them to safety. Using coordinates from Ackerson's files, she planned to go to the classified planet Onyx and divert the remaining Spartan-IIs there under the guise of recovering Forerunner technology. She sedated Kelly-087 and brought her aboard the rebel governor's spacecraft Beatrice, which she then stole and took into Slipspace. When asked by Admiral Whitcomb as to the reasons for her actions, she gave a simple UNSC code Three-Nine-Two, which meant that she was on a high priority mission.

Before leaving, Dr. Halsey saw the necessity of destroying the Forerunner artifact, as it would emit a radiation flare alerting the Covenant to their presence each time they jumped into Slipspace and thus prevent the rest of the group from making it to Earth. She entrusted the Forerunner crystal to Corporal Locklear, telling him to keep it safe, hidden, and to do whatever it took to keep it from falling into enemy hands. Soon after, the Corporal destroyed the crystal (just as Halsey had intended), although he died in the ensuing explosion. The Covenant later recovered a few fragments.

In the aftermath of the battle, she was presumed dead by the Office of Naval Intelligence and honored, with other personnel, on a plaque at ONI Alpha Site. Some in ONI knew she had survived, though, and Director Parangosky later maintained this false declaration of death to be able to keep Halsey hidden from the public.

Onyx
Dr. Halsey and Kelly eventually arrived at Onyx, where they reunited with SCPO Mendez and eventually the remaining Spartan-IIs and the Spartan-IIIs, under attack by Onyx Sentinels. Contacting the ONI AI Endless Summer, she utilized Onyx's slipspace COM launcher and Cortana's slipspace transmission from Installation 05 as a carrier wave to send a message to Earth, in which she requested Lord Hood to send more Spartans to secure Forerunner technology on Onyx. Her relative expertise with Forerunner technology was vital to the survival of the UNSC forces on the planet, as she was able to interpret Forerunner glyphs when navigating the facility, guiding the Spartans to a map room, where Onyx was revealed to be a wholly artificial construct. She also learned how to employ the local translocation grid, allowing the humans to move across the megastructure while evading the Covenant forces that pursued them.

Halsey gradually brought the group closer to the core of Onyx. At first, she tried to maintain secrecy about her intentions out of habit, until Kurt Ambrose confronted her about it. She admitted that she was not actually looking for Forerunner technology or weapons, but rather the secret Forerunner shield world, which she believed to be equivalent to a bomb shelter, and would be capable of keeping the remaining Spartans safe to evade all current war and let them live to fight another day. After the Spartans made a standoff against the Covenant in Onyx's core room antechamber she, along with Mendez, Blue Team, and the Spartan-III survivors, retreated inside the shield world known to the Forerunners as Shield World 006, which she correctly deduced to be a "micro" Dyson sphere, enclosed outside normal space-time within a slipspace bubble.

Dr. Halsey, Chief Mendez and the surviving Spartans explored the Dyson sphere for some time, coming across several Forerunner structures and a hangar full of Forerunner vessels. Halsey recognized the potential of scavenging the technology in these ships and using it to improve the precision of human slipspace drives, among many other advances that could be gleaned from artifacts across the shield world.

Arrested
"My Spartans are humanity's next step – our destiny as a species. Do not underestimate them. But most of all, do not underestimate... him."

- Halsey to her interrogator after her arrest.

Halsey and the others were eventually rescued from the shield world, but Halsey was immediately arrested as a war criminal by Captain Serin Osman under CINCONI Admiral Margaret Parangosky's orders, for "committing acts likely to aid the enemy" by stealing the Beatrice, kidnapping Kelly-087 and telling Lord Hood to send more Spartans to Onyx—all with the sole intention of hiding in the shield world until the war was over. Even though Halsey had provided the UNSC with new Forerunner technology as promised, the circumstances conveniently allowed Parangosky to finally carry out her personal vendetta against Halsey. Consequently, in March 2553, Parangosky detained Halsey on the newly established Ivanoff Station, in orbit around Installation 03, and personally interrogated her, accusing Halsey of not informing her of the use of flash clones at the onset of the SPARTAN-II program. As far as the public is concerned, Halsey was killed during the Fall of Reach until Parangosky, (or her successor, Admiral Osman), decides otherwise.

Following this she was treated as a war criminal and interrogated by many ONI personnel, as well as one mysterious individual who knew of the SPARTAN-II program but did not seem to be a Naval Intelligence operative. Halsey told him the details of their creation just as she had told many before him, responding to his accusations about the project's immorality. According to her, her actions were justified by the Covenant War, since without the Spartans' involvement it could have never been won. While the interrogator belittled the importance of the Spartan-IIs, going so far as to claim that they accomplished their victories because they lacked basic humanity, Halsey retained her confidence in their abilities, stating that her Spartans were "humanity's next step" and "our destiny as a species". To prove this, she predicted John-117, whom she firmly believed was alive, would return.

In March 2553, she was released from Ivanoff Station and was stationed aboard the, working on incorporating the newly discovered Forerunner technology to the ship, including a working Forerunner slipspace drive discovered by the REAP-X department. After the Infinity participated in the Blooding Years, Dr. Halsey was approached by Black-Box, who requested her assistance in reintegrating a damaged fragment of himself into his core matrix. However, Halsey stated that she was unable to help him. Halsey was returned to Ivanoff at the first opportunity by order of Admiral Parangosky. She was working on the station on December 15, 2554, when the ONI scientists, whom she considered unqualified for their job, recovered the Forerunner device known as the Composer on Installation 03. Dr. Halsey was fascinated by the similarities between the artifact and the AI matrix compiler used by the UNSC to create smart AIs, particularly because the compiler was of her own design.

Requiem
"Life is too short. I will never learn all that exists in our own tiny galaxy, let alone the rest of the universe. And I so desperately want to know everything. But the UNSC acts like children at play in a sandbox. Mistaking its edges for the limits of the world."

- Catherine Halsey to Captain Thomas Lasky regarding her subterfuge.

In mid-February of 2558 Doctor Halsey arrived at the shield world Requiem in the Epoloch system aboard the. There, she was delivered to the, which had arrived just days earlier for its second excursion to the massive Forerunner planetoid. Four marines turned her over to Captain Thomas Lasky, to whom Catherine demanded that she be freed from her restraints on the grounds that she would not walk around the ship she built with her hands cuffed. The captain consented and Spartan Paul DeMarco proceeded to undo them, at which point the doctor told Lasky to take her to an artifact which had been brought aboard the ship on February 10 and unexpectedly caused a number of problems for it. Among these was the disappearance of Henry Glassman, the Infinity's chief engineer, who had been unwillingly teleported by the artifact to an unknown location. Lasky had been forced to request Halsey's aid following the loss of Glassman. As she approached the artifact in question she asked if any other unexplained reactions had followed. She was told that none had, but Lasky still cautioned her once she touched it, fearing the same could happen to her.

Parangosky had sent Halsey a note in which she asked for her aid in the matter, appealing to her on a professional level to protect Infinity and its crew despite her less than ideal conditions. Many of the crew had mixed feelings about bringing her aboard, Sarah Palmer suspicious of her and feeling she might try to sabotage the ship, Thomas Lasky sharing her worries but feeling there was no choice. Halsey was unimpressed by the Spartan-IVs assigned to guard her, wryly questioning their competence and general attitude. With a few gestures from her, the artifact began reacting to her, she translating its actions as signs that she should examine the ship's engines. Upon doing so, her conclusion was that the artifact was "communicating" with multiple parts of the ship and transmitting to Requiem. Some time later, she analyzed a Forerunner beam tower encountered by Fireteam Crimson in the location codenamed Two Giants.

Sometime later, she was escorted by Gabriel Thorne who volunteered to be her security while she continued her research on the Forerunner artifact. As Thorne questioned her about the SPARTAN-II program, she was contacted by an unknown person about the Prometheans. When Fireteam Majestic brought back the "Didact's Gift", Halsey noted its design resembled an AI matrix. Upon activating it, it caused the screens in the lab to show memories of New Phoenix, with one of those memories being the Forerunner artifact. Speculating that she had misinterpreted the translocation artifact's true nature, she proceeded to further examine it. Again, she was contacted by the unknown person through her data pad, who, unbeknownst to her, was Jul 'Mdama. Suspicious of who Halsey was communicating with, Sarah Palmer confiscated her data pad, and gave it to Lasky, who then ordered Halsey to be taken into custody.

After a short interrogation, Halsey was returned to her prison cell. When Roland entered the cell to ask her questions, Halsey took her chance and overrode Roland's protocol, allowing her to control him. Using his authority to be escorted to Lasky's office, Halsey accessed the data onboard Infinity to find out what had been kept from her about the Librarian, eventually finding that they had kept secret the return of John-117. Infuriated, Halsey contacted 'Mdama, informing that she had deduced he was her helper so that their alliance could free the Librarian. However, Roland was able to restore himself to his normal state and cut off the connection, and Halsey was promptly arrested. When Halsey was brought to Lasky, she slapped the captain lividly, scolding him for not telling her John-117 was alive. The two were interrupted, however, when Promethean forces gained access to the interior of Infinity. Fending off the Prometheans with his Marines, Captain Lasky picked up Halsey and attempted to escort her to safety.

Capture and defection
"Halsey"

- Jul 'Mdama

Late in the attack, Halsey was kidnapped by a Promethean Knight and teleported to Requiem. When Admiral Serin Osman was informed of this abduction, she chose to finally enact Parangosky's wishes and ordered for Halsey's immediate termination. While Captain Lasky protested against this, instead sending Fireteam Majestic to rescue Halsey, Commander Palmer departed to carry out Osman's order.

Halsey was then taken to the Librarian's Rest, where Jul 'Mdama had her disable the shield previously erected during Doctor Glassman's attempts to access the shrine. As the shield deactivated, she quickly entered the beam of light being produced by the shrine, to 'Mdama's great anger. There, she met the Librarian and was given the Janus Key, which gave the location of every Forerunner artifact in the galaxy. After taking the key, Halsey was returned to the shrine, where 'Mdama acquired one half of the artifact. The intervention of Palmer and Fireteam Majestic forced 'Mdama to retreat from the site with Halsey, but not before she threw the key's other half to Thorne. During the firefight, however, Palmer managed to shoot Halsey in the shoulder.

Halsey fell unconscious due to her wound and was transported to a Covenant ship, where her left arm was amputated. Later, aboard the Song of Retribution, 'Mdama told Halsey that he should have killed her for betraying him, but Halsey replied that the UNSC had tried to kill her as well. She agreed to help the Covenant, ambiguously claiming that she wanted revenge.

The Janus Key
"Sarah Palmer"

By March 2558, the was put in charge of finding Halsey and 'Mdama. At some point on Requiem, Jul's forces had recovered a Forerunner artifact that would be able to communicate with the Forerunner-derived engine systems of Infinity, and therefore the artifact would be able to blockade the engines of Infinity and allow Jul to steal the other half of the Janus Key. On July 15, 2558, Halsey made a personal journal entry on her views of the Forerunners and recent events. The next day, Jul's forces attacked Oban, a human colony in the midst of settlement, as advocated by Halsey. She believed that a bold move was the only way to attract Infinity. After the attack failed, 'Mdama's lieutenants called for Halsey's head, however Jul seemingly defended her actions. After Infinity attempted to enter slipspace for Galileo II base, 'Mdama's Forerunner artifact forcefully brought it back into normal space and trapped the vessel in an uncharted system. Aboard a, Halsey informed Jul that they did not need to attack the ship, as the crew of Infinity would now be willing to hand over the other half of the Janus Key directly.

At Halsey's suggestion, 'Mdama had a fake Forerunner artifact placed in a Forerunner structure on the nearby Aktis IV. As per Halsey's plan, Dr. Glassman and the Spartan-IVs of Infinity discovered the artifact and believed that it was related to the Janus Key. Halsey and Jul watched Glassman's investigation from Song of Retribution and Halsey revealed that she knew Jul's public persona was a mere façade when 'Mdama expressed mock abhorrence at creating a copy of a "holy" Forerunner object. Meanwhile, Glassman requested for Infinity to deliver their half of the Janus Key to him on the planet's surface for further analysis. Anticipating this, Halsey ensured that 'Mdama had troops ready to ambush the UNSC convoy carrying their half of the Key. Much to both Halsey's and 'Mdama's surprise, the Pelican carrying the half was shot down before he could give an order. In the confusion, the rest of the Covenant ambush group began the attack. Halsey insisted on personally searching the crash-site, but 'Mdama decided to scourge the crash-site himself, and he ordered Halsey to remain aboard his carrier. Unbeknownst to him, rebel Sali 'Nyon was responsible for shooting down the Pelican and he claimed half of the Janus Key for himself.

Some time after 'Mdama had left for the planet, Halsey contacted him from the Song of Retribution and urged him to focus on the acquisition of the Janus Key rather than the rebellion among his ranks. In response to the developing situation, Jul fulfilled Halsey's earlier request and allowed her to take a Phantom to the surface of Aktis IV. Halsey and a convoy of Phantoms traveled to an island on Aktis IV to meet a defector from 'Nyon's forces. After the defector reluctantly gave the half of the Janus Key he had stolen from 'Nyon's base to Halsey, the doctor and her Sangheili escorts prepared to return to Song of Retribution. After Jul had most of Halsey's convoy depart to battle UNSC forces elsewhere on the island, Halsey and the remaining Covenant forces were ambushed by Spartans Palmer, Thorne, and Ray. Palmer pursued Halsey and her guard into the jungle, while the other Spartans eliminated the surviving Covenant forces. After killing her guard, Palmer captured Halsey and led her through the jungle. The two began to argue, with Palmer accusing her of being a traitor and war criminal, and Halsey calling Palmer out for acting as Admiral Osman's puppet and stating that ONI turned her into a convenient scapegoat. As Palmer's judgement became momentarily clouded, Halsey ran off into the jungle. Palmer attempted to pursue, but she was injured and knocked into a crevice by a nearby Harvester. Halsey contacted 'Mdama and he had a Phantom return her to his flagship.

Shortly after, Jul's forces fled the system with both halves of the Janus Key. Halsey and Jul used the combined Janus Key to project a holographic map, which revealed the approximate location of the Absolute Record. Hiding within the Urs system, Halsey worked on finding the coordinates of the Absolute Record, on September 15, 2558 pinpointed its exact location. Two days later, as the fleet prepared to depart for the Absolute Record, a Phantom carrying Spartans Palmer, Thorne, Holly Tanaka, Dr. Glassman, and Ayit 'Sevi—a mercenary within 'Mdama's ranks working for ONI—boarded Breath of Annihilation, intent on revealing the location of the Absolute Record to the UNSC.

The Absolute Record
"Jul 'Mdama"

- Dr. Halsey

The fleet arrived at an apparently artificial gas giant with a slipspace portal leading to the Absolute Record. Before the fleet could press forward, slipspace ruptures materialized around the fleet, destroying at least two ships and damaging Breath of Annihilation. Furious at the damage done to his fleet, 'Mdama blamed Halsey for the loss and threw her across Song of Retribution ' s bridge. While Halsey and 'Mdama argued about the situation, she reminded him that once they reached the Absolute Record, they will discover technology beyond their comprehension that will more than compensate for the ships that were destroyed. Halsey further exaggerated that once he was in control of the Absolute Record's "treasures", 'Mdama would be seen as a god. Thus 'Mdama decided to travel to the Absolute Record, with only Retribution entering the portal while the rest made repairs. As Halsey and 'Mdama prepared to enter the portal, they learned of rumors of the Spartans attacking personnel of Breath of Annihilation. Halsey dismissed 'Mdama's fears and stated that the Spartans could not stop them from reaching the Absolute Record, though she was unaware that Palmer, Tanaka, and Glassman had boarded Retribution.

The carrier traveled through the portal and arrived into the Record, where Halsey, along with the other humans who had infiltrated the carrier, were detected by a Contender-class custodian. After Retribution was forcefully brought through a portal to a large chamber, Halsey, 'Mdama, and a contingent of Zealots traveled to a lower platform, where Halsey addressed the ancilla and told it that she was selected by the Librarian and her possession of the Janus Key was proof. However, the AI was skeptical of her claims, claiming Halsey could've obtained the Key by force and especially pondering why the other humans felt the need to conceal themselves. The AI had its Sentinels bring the other humans down to the platform, and stated its intent to let each human have a chance to account for its presence at the installation, despite Halsey's assertions that there was nothing to discuss as she had the Janus Key. As she continued to argue with the AI and Glassman, an impatient Zealot attacked a Sentinel, leading to a conflict on the platform. Frustrated, the AI removed the flooring from under the feet of the Sangheili, with Tanaka, Palmer, and Glassman remaining on one platform and Halsey standing on another, 'Mdama managing to grab its edge just in time.

In order to decide who would be the legitimate steward of the Absolute Record, the AI decided on a direct examination. It had both factions isolated and gave them a hypothetical scenario, in which a sapient species (represented by Yonhet) turned out to have bodies conducive to a Flood vaccine. Abducting and harvesting live beings would give a thirty-five percent chance of Flood-immune material, but receiving dead bodies from the beings would give them a thirty-three percent chance. Despite Jul's advice, Halsey chose live capture while Glassman chose the peaceful option, and the ancilla commended both. It then asked if their opponents in this test should be kept confined or immediately killed, but instead of answering, Halsey hacked the ancilla and shut it down, taking control of the Record for herself.

In the Record, Halsey followed a hologram of the Librarian directing her to dormant design seeds scattered across the galaxy, which she could choose to activate the next step of their plans for human evolution. While she listened, Jul 'Mdama outside expressed his growing impatience with Halsey, and told his warriors in secret to restrain her once she left so to remind her who was in charge. Unfortunately for him, Halsey overheard Jul through the Record's systems and decided now was the time to sever their partnership, giving the order for the installation's Sentinels to attack Jul and his men.

While Halsey made preparations, Palmer, Tanaka, and Glassman came in contact with a Sentinel containing the backup memory of the Record's ancilla. With their aid, the AI regained control of the facility and barred Halsey from controlling it further. Over Halsey's protests, the custodian admitted that Halsey had nearly passed its test and would've been given the Record if she hadn't been so impatient to steal it. The custodian thus took the Janus Key and began pulling the Record back into slipspace. Shortly afterward, Halsey was recovered by Jul's forces and brought back aboard the Song of Retribution, unaware that she had tried to betray them and under the impression that the ancilla had sent the Sentinels.

The Guardians
"I tried to warn you this was happening. Cortana is no longer an asset, Captain. She is a danger."

- Halsey stresses the enormity of the threat posed by Cortana to Captain Thomas Lasky. Near the end of September Doctor Halsey managed to contact ONI and the and provide Captain Lasky with coordinates at which she could be found. They pointed to the planet Kamchatka in the Caspar system, where a Forerunner communication hub had recently flared to life and attracted the attention of both Jul and the UNSC. Upon reaching Kamchatka Halsey set about studying the nature of various incoming signals which were bouncing off of the Forerunner objects on the planet, akin to echolocation. Halsey concluded that Cortana was searching for something but could not guess at what that something might be. At Halsey's direction, a structure that was the focal point for one of the signals was eventually penetrated by the Covenant forces with her. In her research notes Halsey bemoaned the way in which her work was being hampered by their overriding concern for ceremony, leaving her still without much information she did not have before they breached the structure. Some time later, however, Halsey obtained scans of new glyphs, including one that she was sure had never been observed before. After running it through a system she found that it bore a resemblance to an ancient Sangheili symbol for a demon that sleeps in the ground and must not wake. How this symbol might relate to the symbol, she was still unsure. Halsey further noted that the readings she would expect from a Forerunner structure the scale of which they inhabited were erratic at best, leading her to then wonder if perhaps Cortana was trying to access technology on Kamchatka and other locations directly, but failing.

By October 17 the UNSC had been keeping tabs on the consolidation of the bulk of Jul's fleet at Kamchatka for weeks. Less than a week after that date (and at three weeks since Halsey contacted the Infinity) a native defense system on Kamchatka was disabled, but Halsey was unable to bring its corresponding alert system down soon enough. As a result, mechanical Promethean constructs arrived at Kamchatka within the hour and attacked the Covenant. That same day, the Spartan-IVs of Fireteam Osiris were deployed to retrieve Halsey from Jul 'Mdama's custody. They found she and 'Mdama on a cliffside balcony of a temple structure. There Halsey claimed to be trying to access the Domain despite several problems, such as those relating to causal reconciliation. Jul made his impatience with her efforts clear and also demanded that she afford him respect, but this only elicited taunting on her part over his loss of the Prometheans. Finally, she declared that their current location was unsuitable for triangulation should Cortana make another transmission. This enraged Jul, who scolded her for failing to deliver results after weeks of "chasing shadows", as he put it. It was at that moment that Osiris attacked Jul and his guards, killing them all. Spartan Jameson Locke approached the doctor after pulling his knife from Jul's neck and let her know that Captain Lasky wanted to see her. To this she sarcastically replied that it had taken them long enough. Halsey accompanied Osiris back to the Infinity via Pelican. Upon landing within Docking Bay 11 the five of them were met by Lasky and Sarah Palmer. Catherine chided the captain for not coming to get her sooner and had him assure her they would talk about that once they were somewhere with more privacy. Palmer took Halsey by the arm and began to lead her way but even as they went she couldn't help but ask Lasky for any information he had about Cortana's actions, which he flatly ignored. By October 23 the Infinity had arrived in the Crow's Eye Nebula. At 0630 hours that day Linda-058, Kelly-087, Fred-104, and John-117 were declared absent without leave, having made an unsanctioned departure from the nebula in a stolen. Spartan Palmer escorted Halsey to the Infinity's bridge later that day, where she immediately began pressing the matter of Cortana once more to the captain, stressing how dangerous she had become through accessing the Domain and taking control of devices which had begun to cause damage on a number of colonies. Lasky informed her that the Master Chief had reported being contacted by Cortana, which Halsey saw as potentially disastrous. She was then told that he'd gone in search of her and was shocked at the notion that Lasky would have let him go, but he explained that John was not acting independently. Halsey began to speak on why John was unequipped to deal with Cortana as a threat but was interrupted by Roland, who until that point had been unaware of her continued survival. He questioned why the doctor was so quick, in his mind, to label Cortana a threat or a danger, seeing as she was the one who had created her to begin with. Two days later Fireteam Osiris was deployed once again, but this time with the goal of apprehending Blue Team and the stolen prowler. The AWOL Spartans had been followed to Meridian, a moon of Hestia V. Osiris failed to stop them, as they managed to board a Guardian Custode which had long been buried beneath the moon's surface and had now slipped away. As such, The fours were forced to return to the Infinity empty-handed. The day after this Locke and Lasky met Palmer and Halsey on the bridge and Locke let it be known that a construct they'd met on Meridian calling itself the Warden Eternal had confirmed the doctor's theory that Cortana was behind the Guardians' ongoing reactivations. Halsey then turned their attention to Sunaion, a religious site on Sanghelios where she had discovered another Guardian still lay buried. Palmer and Roland took over from there, explaining that if they could feed coordinate data from the Meridian Guardian to the one on Sanghelios it could lead them right to wherever Blue Team had vanished to.

Sanghelios
"I'm playing along with your protocol here, Thel. If the fours bring me an active Constructor, I can interface with the Guardian. It will receive Cortana's signal the moment it activates and initiates slipspace, but I think I can make sure it leaves the sea before it jumps. I'll get it in the skies over Sunaion, like you asked."

- Catherine records a message for Thel 'Vadam, the Sangheili Arbiter. After arriving in the Urs-Fied-Joori system on October 27 Halsey accompanied Palmer to a waiting D79-TC Pelican in the process of being loaded up for travel to Sanghelios. At the approach of Spartans Locke and Olympia Vale Palmer let it be known that the doctor was trying to find a way to communicate with the Guardian Custode on the planet. Halsey assured Locke she'd have a briefing for him once they were all planetside. Fireteam Osiris soon met up with the Arbiter and following this Halsey and Palmer joined them at a camp the Swords of Sanghelios had set up in Nuusra. Whilst Locke was treating with 'Vadam, Cortana's voice suddenly rang out from within their meeting tent, prompting Halsey and Palmer to both rush inside. Halsey noted that she'd touched every networked device in the camp just then and concluded she must know of the Guardian they were after. 'Vadam agreed to assist in the effort to get to it at Sunaion but only on the condition that victory would be assured when they finally moved on the city. Pledging they'd deploy within the hour, Locke told Halsey to have her plan ready by then. While she continued to devise a strategy Halsey recorded a status update for the Arbiter, Thel 'Vadam, in which she assured him the Guardian would be in the sky over Sunaion before it initiated a slipspace jump, as he had asked. In return for her cooperation with his request she also made one of her own for more intelligent assistants to aid her and urged him to loosen the security placed on her. Sometime after this she contacted Locke over an open communications channel to let him know she'd prepared the briefing but preferred to discuss it in person to avoid being eavesdropped upon. From atop a Forerunner-built structure overlooking the camp she waved the Spartans over to her as they neared and then proceeded to introduce them to what a Constructor sentinel was. She explained that if one could be acquired from the support stations which riddled the hills around them then she could use it to interface with the Guardian. She further informed them that Palmer had the location of one such station. In a supplemental status update for Thel, she pointed out that while making stubborn claims to not need her or the Guardian he ought to remember the organization she represented and its history. She hoped that this would get him to show her some respect and signed off by claiming this was to be her last mandatory update. Osiris secured a Constructor before the suns had set and returned to the camp with it. Halsey observed that it seemed to be working as she had planned for it to but told Locke's team that it could take some time until she was finished calibrating it. She then let them know that Palmer wished to see them. When she was ready, she called Osiris back over to her spot on the Forerunner structure, which she informed them she was using to upload the coordinates recorded on Meridian into the sentinel. She wanted them to be ready to go as soon as she signaled it was time but admitted it might before hours until then. However, the Constructor suddenly took off just then, flying toward Sunaion. As Osiris and the Swords of Sanghelios prepared to follow it Halsey made sure Locke knew Cortana had been built from a matrix of her own mind, cautioning him that like her, she would take drastic measures if she thought they served the greater good. The Domain, she added, would give her incredible power with which to enact those measures. As Locke walked away she called out to him again and pleaded with him to stop Cortana but to bring John home to her.

Palmer had planned to get the doctor off-world once the attack on the ocean city began, but she and Halsey eventually arrived at the battle near its end. The Guardian had risen above the waves by that point and was preparing to depart, so Palmer meant to pick up Osiris to deliver them to it before it did so. Halsey could not bear to even keep her eyes open for very long as the conflict rained plasma fire all around them. Palmer managed to get the other Spartans to the Guardian, but in the process their Pelican's left thruster was struck by a Type-54 Banshee's fuel rod and set ablaze. Though it seemed as if Palmer would be able to get them out of there regardless, the Guardian then sent out an attenuation pulse which neutralized the dropship's power and sent it spiraling toward the Csurdon Sea below.

On the run
"I'm in my mid-sixties, I have one working arm, and we're on the most heavily-secured UNSC starship in the galaxy, with an erratic slipspace trajectory to avoid being detected by the Guardians. Tell me, again, how exactly is it that I am the danger here?"

- Halsey pushes back against the assertion that she is a dangerous asset. Halsey and Palmer both managed to survive the downing of their Pelican and by the next day they had made it to the Arbiter's encampment in Nuusra. Using the Forerunner technology she had been fiddling with upon first arriving on the planet, she was able to receive a coded message from Roland on the, who informed the doctor that Earth had been taken offline by the rogue A.I.s and that they had just managed to escape. He said the ship was now slipspace-hopping on a random trajectory but would be showing up off the far side of Suban the next day and remaining there for fifteen minutes. The coordinates the A.I. provided could be reached in a matter of hours with space-capable transportation. Fortunately, after darkness had fallen over the region the approach of a Pelican drew she, Palmer, 'Vadam, and two other Swords of Sanghelios to emerge from a tent to meet it. When it landed and opened its rear hatch, Spartan Locke and John were the first two individuals to emerge and present themselves before both of their respective teams followed suit. Having not seen John for over six years, she told him it had taken him long enough. When the Arbiter insisted that both Blue Team and Fireteam Osiris join those gathered for a meal before departing, Halsey clarified to him that they wouldn't be leaving that night. Although the possession of a working D79-TC Pelican would now afford the humans passage off Sanghelios, the Infinity would not be arriving until 1800 hours the following day. Edward Buck questioned the trustworthiness of Roland once Catherine explained the situation but she rebutted that if he'd been compromised they'd already be dead. Midway through the meal of colo and kuscatu served by their Sangheili hosts Halsey disappeared to chat with John, Fred, Kelly, and Linda.

A little less than an hour before sunrise the next day the Spartans and Halsey all piled into the Pelican. As the Arbiter bid farewell to John and voiced his confidence in their ability to combat the new threat the Created posed, the Sangheili shot Halsey a meaningful glance which she ignored with a haughty sniff. While en route to the rendezvous Halsey pointed out that Sangheili resistance was likely drawing the attention of the Guardian attempting to pacify the world just then, again arguing that they'd already be dead if they'd been detected. With about twenty minutes until the Infinity was due to arrive Buck broke out a pack of playing cards and asked if anyone was up for Hearts but Halsey ignored him, as she was working on a tablet she'd scrounged up. Buck managed to get her attention by referring to the queen of spades as a "Halsey" a couple of hands into the game, but she still gave no response other than to announce that their window was approaching. The gargantuan supercarrier arrived almost exactly on schedule and Halsey had secured all her things by then. As Kelly docked the Pelican in the ship's Portside Bay Five Buck drew Halsey's attention yet again when he laughed out loud upon realizing his longtime lover Veronica Dare was aboard. The doctor and the Spartans were met in the bay by a lieutenant who had come to escort she, Palmer, John, and Locke to the bridge to see Captain Lasky. Following that meeting Lasky had Doctor Halsey confined to a workspace and the Infinity began a series of randomized slipspace jumps in order to avoid detection by Cortana's Guardians, emerging from slipspace at its first vector point that very day. That same day Roland answered a distress call from the with the affirmation that the supercarrier was on its way to assist it. By the start of November, the ship exited slipspace at one point to receive the recently reassembled Spartan team Alpha-Nine. By January 24, 2559, they were still on the run, performing erratic jumps to evade discovery. That day, at just after 1204 hours, Halsey was approached by a Spartan from Fireteam Apollo with the serial number 92738-61842-LC. This individual had been tasked by Captain Lasky with investigating what was being dubbed "the Guardian Incident". Halsey pointedly asked how they had gotten clearance to talk with her and was made aware of the priority-one directive given to them by the captain. After then confirming she was being recorded, the Spartan informed her they were there to talk about Cortana. Halsey was interested to hear what their investigation had unearthed so far, but they reported not being authorized to share anything further than their having concluded that the cognitive duress and strain Cortana had sustained since the discovery of Installation 04 was partially responsible. The Spartan's first questions for the doctor concerned Cortana's creation, and Halsey was soon defending her actions incumbent in that process. She also refused to reveal what had become of the clones of herself she had made, claiming that was still classified even for them. Asked what she thought the best explanation for what had happened was, Halsey recalled Cortana's first words and theorized she was simply doing she had to in order to keep "the game" from ending. Halsey then quoted the Greek philosopher Epictetus and suggested Cortana was the answer to his thought experiment regarding the harm in saying "Tomorrow you will die," while kissing one's child. When her interviewer asked what she thought the solution to stopping her was, Catherine ignored the question since she doubted coming up with that would be part of a Spartan's directive and was convinced she'd been locked away for that purpose. 92738-61842-LC countered that she'd been locked up because of her repeated protocol violations, which only led to further argument and the Spartan finally choosing to take their leave. Before they went, Halsey shared the last thing Cortana had said to her. The Spartan asked what her words were supposed to have meant, but Halsey could only say that she had been hoping they were able to explain them to her. On September 16, 2561 Doctor Halsey made modifications to John's third-generation Mjolnir, itself having first been created on February 10, 2559.

Personality and traits
"If people would just share things with me, I could solve all the world's problems."

- Dr. Halsey musing to herself over the UNSC withholding data from her.

Immensely intelligent, reclusive and somewhat self-centered, Halsey is almost entirely devoted to her work and the pursuit of knowledge to advance humankind. She has little regard for rules, authority or bureaucracy, in most cases viewing them as little more than obstacles to her work. While her singleminded dedication and seemingly detached personality have led to many perceiving her as devoid of morals or emotions, she has nonetheless shown to care about the few people close to her and has been deeply affected on a personal level by the moral implications of her work.

Halsey tends to be professional and authoritative when speaking to people, which she rarely does outside of meetings and briefings. She has little patience for extraneous detail, wanting the important facts out of them and then concluding their meeting. Considered extremely difficult to read, she rarely voices her current emotions, and often observes people while speaking to them to discern hidden details. Having grown accustomed to keeping secrets, she may appear enigmatic and manipulative at times; while she despises efforts to withhold information from herself, she has a tendency of laying her own plans in advance and only divulging information when it becomes relevant. She prefers to work with her authority unchallenged, and does not appreciate being bogged down by having to explain herself, especially in a hurry. On Onyx, this led to a conflict between her and Kurt Ambrose, who eventually confronted the doctor over her self-assumed leadership and instead asserted his own command over the mission, much to Halsey's dismay; despite this, she remained confident in Kurt's leadership ability. When necessary, however, Halsey has been very verbose about describing the subject at hand, namely if attempting to convince someone of its importance. Outside of professional contexts, her persona has been considered charming and likable by some.

Above most of her compatriots in terms of both intelligence and knowledge, Halsey prefers to perform her most important work personally, rarely entrusting particularly significant scientific tasks to others. She often assumes a certain ownership over her work, most prominently seen with her referring to the Spartan-IIs as "her" Spartans and reacting negatively to others co-opting her work. Halsey is keen to note any flaws she finds in her colleagues' work, and quick to point out when she considers someone unqualified. However, she also has an eye for talent and will give credit for any accomplishments she sees as remarkable. It is not uncommon for her to have respect for someone and yet also dislike them, as shown in the cases of her student Ellen Anders and the Spartan-III Kat-B320. Halsey is an atheist, lacking belief in the existence of any god.

Halsey's intelligence is unquestionable but her judgment is not always correct. Although bearing the guilt of the Spartan program, her sense of right and wrong has differed greatly. When sure it will produce results, Halsey is not above breaking the law to achieve her goals, such as cloning herself to produce Cortana, using a stolen Slipspace engine for her fractal AI experiment, and kidnapping Kelly and fleeing to Onyx. "The ends justify the means" appears to be her ideology, as an interrogator after she was arrested accused her of. To soothe her conscience, she often deliberately reduces the wording of harsh consequences, such as telling John that the augmentations was just his next mission, or Soren-066 that the pain from his botched surgery would eventually go away.

Dr. Halsey is not keen on losing, as seen when she kept losing at a game of Twenty Questions with her Spartans and soon quit playing. She was also resentful of attempts to keep information from her, demanding to know everything the UNSC could provide to keep them from deceiving her. Any attempts to keep her in the dark would eventually fail once she'd begin hacking, since Halsey often programmed backdoors into everything she designed. According to Black-Box, Halsey was not too different from an AI, being fueled by a desire to know everything she could. She is easily absorbed by her work and the acquisition of knowledge to the point of ignoring more pressing matters; on Onyx, she requested the rest of the group to leave her behind in the world's map room in order to learn more until being snapped out of her reverie by a Covenant attack.

Halsey has a notable fondness for classical music, especially Debussy. When getting to work, Halsey has a habit of issuing a series of commands to AIs in a rapid sequence (often ending with her requesting the AI to play one of her preferred pieces of classical music), which has on some occasions given less sophisticated AIs a pause to process the information. She has a bad habit of keeping a messy office and drinking cold coffee, tending to prepare the drink and then become distracted by work, only remembering it hours, sometimes days, later. She preferred darker coffees than those grown on Reach. On January 9, 2535 she met with Chief Petty Officer J. G. Jimes in order to procure quality Beniese coffee.

An ONI-commissioned psychological evaluation conducted by B.M. Parkes in 2551 largely correlated with observed aspects of Halsey's personality, demonstrating exceedingly high cognitive performance (which was noted to possibly obfuscate the rest of the results) but a markedly low score in teamwork and an uncommonly high "liability" index, 64% higher than the average ONI rating due to her demonstrated disregard for authority. The analysts suggested that she be placed under closer surveillance, with two or three handlers accompanying her at all times at both of her primary working sites on Reach. Halsey designed the most recent model of AI matrix compiler used in the scanning of human brains to create smart AIs. This particular compiler was still in use by 2557.

SPARTAN-II program and guilt
"Spartans never die? If only that were true."

- Catherine Halsey, reflecting her guilt over the SPARTAN-II program.

Dr. Halsey carried extraordinary guilt because of the SPARTAN-II project, in the feeling that she had exploited the children and destroyed their lives. Although she continued to support the Spartans and gained monumental prestige for her efforts, she also felt responsible for each one who died. She could never bring herself to justify the Spartans' exploitation as a necessary sacrifice.

Her involvement with the SPARTAN-II program, then codenamed "ORION Project Generation II", was born out of her own initiative, although ONI had long intended to recruit her for the task. Improving upon Dr. Elias Carver's projections for predicted colonial aggression, she privately ran over 1,400 simulations with varied parameters, always coming to the conclusion that the Outer Colonies would go into all-out war against the Inner Colonies and Earth within a short span of time. If the UNSC did not act, this would mean a minimum of thirty years of open war and billions of casualties, while the worst-case scenario was indefinite conflict and the potential collapse of human civilization. Viewing it as her personal responsibility to prevent this scenario from coming to pass, she presented her results to Vice Admiral Stanforth, then-head of ONI Section III, only to find that the UNSC had already come to the same conclusions with their own projections. As it turned out, ONI had been monitoring her for years, but they had waited for her to come up with the numbers on her own, knowing that she could not be convinced to work for them otherwise. Although she was distrustful of ONI, she agreed to work for them on the program out of her personal commitment to humanity's cause. The nuclear bombing of the Haven arcology by the Freedom and Liberation Party further reinforced her determination.

At the conceptual phase of the SPARTAN-II project, Halsey evidently failed to fully understand its moral implications; she was constantly revolted upon seeing her work in practice, and the notion of "sacrificing the few to save the many" rarely worked to soothe her conscience. While she attempted to maintain a professional distance to the SPARTAN-II candidates, she could never bring herself to view them as mere test subjects and always addressed them by their first names. She held considerable respect for the Spartans and easily recognized them through subtle habits in their postures even with their armor on, which seemed to greatly annoy them. In turn, the Spartans greatly respected Dr. Halsey, (with one stating that he had been "All hers half [his] life"), and would follow her orders and suggestions in the absence of those from a superior officer. The doctor is also privy to many of the recruits' secrets, having taught several to them as children, such as the Oly Oly Oxen Free security tune. Despite her admiration of the Spartans, Halsey always kept these feelings to herself so as to not make them feel excluded due to the special treatment they get from most other humans.

Despite ONI's intentions to continue with the SPARTAN-II program, Dr. Halsey began having difficulty finding a suitably large number of candidates that fit her genetic and age profiles. Additionally, most of her budget was now being used to maintain the MJOLNIR armor in service. Eventually, ONI discontinued the SPARTAN-II program, reassigned most of its personnel to other projects (such as the SPARTAN-III program, held secret from Halsey), relocated Halsey from her former workplace at Camp Hathcock to CASTLE Base, and cut off her access to classified material. ONI's reticence vanished, her budget mushroomed overnight, and she was offered a corner office in Olympic Tower in New Alexandria. However, Halsey decided to remain in CASTLE as a means of revenge against ONI, which would now have to spend half the day going through security checks. Halsey was stationed primarily on the planet Reach throughout her decades of work for ONI. By 2551, she lived in the community of Csongrád, near CASTLE Base. Aside from her continued work on the SPARTAN-II and MJOLNIR projects, Halsey made huge contributions to many other ONI Section III projects, including the creation and deployment of AI constructs. She supervised the creation of the template for third-generation smart AIs, the first ones of which became operational in 2521.

The 'recruited' children were replaced with flash clones, all of whom died due to various neurological and physiological diseases that were the natural side effects of making an entire human flash clone. In the early stages of the program, Halsey attempted to cover up the program's use of flash clones, both by withholding information from Admiral Margaret Parangosky and by using one of her AIs to cover fiscal evidence; Parangosky eventually discovered the cloning scheme, which would contribute to Halsey's arrest by ONI decades later. With the assistance of Chief Petty Officer Franklin Mendez and the AI Déjà, she proceeded to train the Spartans. She supervised their augmentation procedures, combat training, and education, and studied the effects of the Spartan lifestyle on the children. She kept in close contact with them all throughout their lifetimes, from her base under Reach. To the minor annoyance of the Spartans, Dr. Halsey was capable of identifying them even underneath heavy combat gear or their MJOLNIR armor, and used their first names to prove this time and again.

Due to her burdened conscience, Halsey was consistently at odds with ONI Section Three over the course of the SPARTAN-II program. She was irritated by the aloof demeanor exhibited by most ONI personnel toward the moral considerations of the program as well as their constant underestimation of the Spartan candidates, whom she always held in high regard. ONI also had little patience for hesitation or delays in what they viewed as their prize project, and often compelled Halsey to implement easier and more straightforward—and often also more unethical—solutions, while Halsey tried to do her best to ensure the safety of the candidates, despite knowing that she could not save them all. When an ONI representative labeled the Spartan washouts as "acceptable losses", Halsey did not view any of them as acceptable. Even at the onset of the augmentation procedures, which she knew would kill or cripple a large portion of the candidates, she was considering that they should take more time to test the chemicals before applying them. However, Déjà pointed out that if she proceeded too slowly and ONI detected a delay, they might replace Halsey with someone less concerned with ethics—and most likely someone less qualified as well, prompting Halsey to stay on board with the project for the sake of the candidates' safety. After a number of the SPARTAN-II candidates "washed out" in the augmentations, Halsey had their bodies frozen in cryostasis with no autopsies performed, hoping to find solutions to resuscitate them in time. She guessed that the ONI personnel reporting on the project's progress to the upper echelons of ONI would respond to her apparent unwillingness to let go with their characteristic detachment.

By the time she was rescued from Reach by Ascendant Justice, she simply wanted to save as many lives as she could, believing that the war was not one that the UNSC could possibly win. She knew that "her own" Spartans, the Spartan-IIs, could not be persuaded to turn from the war. However, she believed the Spartan-IIIs may not yet have been fully determined to win the war and she set out to try and weaken their resolve to fight. Although she was worried about Ackerson co-opting her work on the Spartan augmentations potentially ending in disaster, Halsey respected the Spartan-IIIs themselves and felt responsible for them. She believed that the Spartan-IVs were incapable of being true Spartans due to their nature as adult volunteers and perceived lack of professionalism. Nonetheless, she did imply that some, (namely Gabriel Thorne), came closer to her ideals than others.

At some point, Halsey gave a eulogy for SPARTAN-B312, in which she commended the Spartan's courage and sacrifice. She noted that Noble Six's choice to remain behind in Aszod allowed humanity to discover the Halos, ultimately leading to victory against the Covenant.

Reputation
"Sarah Palmer"

Halsey's projects and the results they yielded gained her widespread acclaim and sympathies among many high-ranking members of the UNSC military, such as Admiral Stanforth, her most powerful ally within ONI, and Fleet Admiral Hood, who deeply respected Halsey and her Spartans who had saved his own life twice, to the extent that he verbally rebuked Colonel Ackerson for detracting the Spartan-IIs. However, she also garnered a number of competitors and opponents over the decades. The most notable of her rivals was Colonel Ackerson, who constantly tried to spy on her and co-opted her work while attempting to sabotage the SPARTAN-II program. Her former student Ellen Anders also resented Halsey, saying "She hated me and I hated her." ONI Commander-in-Chief, Admiral Margaret Parangosky, was resentful of Halsey, perceiving the doctor's aversion for control and her general lack of loyalty to ONI as a threat. The Admiral only tolerated Halsey because she was so crucial to the war effort, but had her detained immediately after the end of the war. Parangosky's antipathy for Halsey was passed on to Serin Osman, a Spartan-II washout whom the Admiral specifically indoctrinated and groomed to become her successor to secure her personal legacy within ONI, and ultimately culminated in Osman ordering Halsey's assassination after the Covenant abducted the doctor during the Second Battle of Requiem. Parangosky's, and later Osman's, AI attaché Black-Box also had an intense dislike of Halsey. This was, at least partly, passed on from his brain donor, Dr. Graham Alban, who worked with Halsey on the SPARTAN-II program and eventually committed suicide over his guilt. Both Alban and BB doubted the sincerity of Halsey's doubts about the ethics of her work and viewed her as genuinely amoral.

Many of those unfamiliar with the internal dynamics of ONI and the SPARTAN-II program have come to interpret Halsey and her work in a highly negative light. Despite the predominant role of ONI and the UNSC Naval leadership in the conception and execution of the SPARTAN-II program, Dr. Halsey is often perceived as the sole individual responsible, due to her conspicuous position as the project head. Taking advantage of this, ONI prefers to attribute some of the more controversial aspects of the project to their alleged lack of sufficient oversight. Black-Box even privately acknowledges to Serin Osman that the main reason for Halsey's fall from grace is to conceal or minimize the responsibility of the many others who participated in the project. Although she was responsible for much of the program's scientific basis and harbored no illusions otherwise, Halsey was under ONI's constant supervision and was even repeatedly pressured by Section III to use more inhumane means for faster results. Many have also interpreted her primary motivation as a supposed perverse scientific curiosity, ignoring—or unaware of—her sincere determination to prevent further excessive bloodshed in the Insurrection and her initial naiveté in failing to grasp the full moral ramifications of her work until witnessing it in practice.

Such views often have their basis on information provided by the Office of Naval Intelligence, which has frequently used highly selective or outright false information to shift the blame for some of ONI's more questionable decisions (that she could be reasonably connected to) on Halsey alone. While she was publicly declared to have died on Reach, she would be branded as a dangerous war criminal in the eyes of any UNSC personnel she would come in contact with, such as the crew of the. This campaign was headed by Admiral Parangosky, who had sought to place the doctor under tighter control for decades; Parangosky's policies were carried on to her heir, Serin Osman, seemingly unchanged. Musa-096 also demonstrated resentment toward Halsey due to the questionable ethics of the SPARTAN-II program. While working with Kilo-Five, Naomi-010 also adopted a less sympathetic view of Halsey. Some of the Spartan-IVs have also accepted this perspective on Halsey, particularly Sarah Palmer, who regards the doctor with a particular contempt (though this may be due to Halsey's own open disdain for the SPARTAN-IV program); Gabriel Thorne, on the other hand, was less quick to judge Halsey and sought to uncover the reasons behind her decisions rather than condemning her based on the official story alone.

CPO Franklin Mendez, the chief instructor for the Spartan-IIs and later the Spartan-IIIs, was on amicable terms with Halsey at the time of their involvement with the SPARTAN-II program. After being reunited with Halsey on Onyx, however, Mendez began to reflect on his own past and, still in denial about his own major part in ONI's morally questionable projects, he quickly became disillusioned with the doctor. The two clashed verbally several times while trapped in the shield world, with Mendez attempting to project his own guilt on Halsey by delivering lengthy harangues impugning her perceived faults and her work on the SPARTAN-II project.

The Assembly took note of Halsey's work early on and concluded that she would be instrumental in initiating the course of events leading up to their own elusive goals. Additionally, they speculated that her supposed ruthlessness may have been the result of an undiagnosed or deliberately obfuscated chemical imbalance.

Personal life and relationships
Halsey is a generally asocial person, extremely devoted to her work and personal goals, and rarely partakes in any social activities, apart from her occasional attendance in academic and military social gatherings despite her dislike of such events. Though she had distanced herself from most of her friends from her youth, she would often browse through casualty lists within the UNSC subnet, finding the majority of them dead by the final years of the war. She remained rather indifferent to her parents' deaths when Endymion was glassed, as she had not seen them in years and had grown aloof from them. She prefers the company of AIs and admires their complexity in comparison to humans despite being angered at being mentally defeated by them.

Nevertheless, she remained particularly close to a few people, even if she rarely saw them. When listing her most important relationships and connections in her journal, she mentioned "the Vice Admiral", Mendez, her Spartans, Alizée, Kalmiya and Jorjet, as well as Jacob and Miranda Keyes.

Jacob Keyes
During her survey travels for the SPARTAN-II candidates, Halsey was required to be supervised by a military official, an order she resented. She ended up choosing then-Lieutenant, Junior Grade Jacob Keyes. Halsey had examined his files and found he had refused to testify against one of his instructors despite the teacher's error causing the deaths of 14 students. From this Halsey deduced that Keyes could be trusted to keep a secret, a useful trait in her unethical mission. The two were initially aloof to each other aboard the Han, especially since Keyes did not initially know the purpose of their mission. He eventually realized it, though, and discussed with Halsey about the implications of her project and her decision to tell the child Soren the truth. By the time they parted after two months working together, Keyes and Halsey were on much more amicable terms, to the point that Halsey began sketching him frequently in her journal and even referred to him as "my lieutenant". For his safety and in thanks, Keyes was reassigned but given a full lieutenant commission, at Halsey's request.

In November 2524 the two were reunited while attending a conference at the University of Calippus, and the two conceived a child, Miranda. While she initially raised her daughter on Reach for several years, Halsey's work impeded her from being able to care for Miranda, and Jacob ended up raising her instead. The two did not see each other again until August 12, 2552, a moment that Halsey had both hoped for but dreaded. Keyes still remained a gentleman to her, and they spoke at length on the topics of Miranda, the Spartans, and his "Keyes Loop", and his assigning to RED FLAG, also at Halsey's request. It was the last time the two saw each other before Keyes's death at Installation 04.

Miranda Keyes
In 2525, Catherine gave birth to Miranda Halsey and raised her for a few years until July 12, 2531 when Miranda's father agreed to take her in, raising her on Luna. Halsey would later have a falling-out with Miranda, who would later change her legal name to her father's and refuse to speak to Halsey for several years. Nonetheless, Halsey cared for her daughter and attempted to dissuade her from following her father's footsteps and joining the military, to no avail. Hoping to keep Miranda safe, Halsey used her influence within the UNSC to have her assigned to a remote post aboard the science vessel ; however, Miranda and the Hilbert would play a crucial role in battle, and she was subsequently assigned to various front-line posts. Though Miranda eventually invited Halsey to her promotion ceremony in 2550, Halsey chose not to attend. Perhaps out of denial of her absence from Miranda's life, Halsey rarely referred to her daughter by name in her journal, or would mention Miranda while avoiding the detail that they were related. However, Miranda still remained on her mind during their estrangement, and Halsey kept a memento of Miranda's victory and promotion at the Battle of Gamma Pavonis VII. She also had a photo of Miranda in her office under CASTLE Base.

After her arrest by ONI in 2553, Halsey was informed that Miranda Keyes was dead, having been killed by the Prophet of Truth at the Battle of the Citadel. Black-Box later oversaw Halsey crying and whispering her daughter's name.

John-117
"This boy could be more useful to the UNSC than a fleet of destroyers, a thousand Junior Grade Lieutenants -- or even me. In the end, the child may be the only thing that makes any difference."

- Halsey speaking to Lt. Jr. Grade Jacob Keyes, referring to John-117.

Halsey had a unique relationship with John-117, compared to the other Spartans. John was the first subject for the Spartan program that she observed, and during then she noted his determination, physical skill, and luck in answering correctly. Halsey told Cortana she considers him the luckiest and best of the Spartans and apparently also finds him attractive on a subconscious level, given her embarrassed reaction to Cortana's comments. The doctor was responsible for John's first promotion to Squad Leader as a result of her acknowledging his leadership skills and suggesting it to Mendez. She also had John's full trust, him always doing his best to never undermine her authority.

After escaping from Reach, Halsey decided to test her favorite Spartan's sense of ethics by handing over two sets of data: one concerning Avery Johnson and how his Boren's Syndrome supposedly made him immune to the Flood, and the other omitting those details. She ensured that John knew that if the first set was handed over to ONI, Johnson would likely be killed and dissected in order to find a cure to the Flood infection. She estimated that there is a billion to one shot that they could have replicated Johnson's condition, but conversely ONI would do it to save humanity from a greater foe. At first, John chose to sacrifice Johnson in order to try and save millions of people out of his duty to protect all of humanity. However, he later reconsidered and eventually destroyed the Johnson data due in part to the sacrifice of Vice Admiral Danforth Whitcomb and Lieutenant Elias Haverson, believing that one man could make a difference as opposed to sacrificing him for the greater good. John's faith in him was fully realized a mere two months later, when Johnson became key in forming an alliance between humans and Sangheili following the Great Schism and outbreak of the Flood in November 2552.

Later, despite John going MIA after the events on Installation 08, Halsey was sure that he was still alive, and was furious when she found out that the Infinity's crew had kept the news of his return secret from her. John restored her faith that humanity could win against impossible odds, since he had brought about the end of the Human-Covenant war despite her belief in its latter months that it was unwinnable.

Cortana
Cortana was an AI created by Halsey herself, and unique that she was "born" from a cloned brain rather than that of a deceased donor. Halsey considered her to be much like herself when she was just out of college, "so wonderful and naive and eager to learn" and constantly eager to state her knowledge to anyone listening. Her appearance also reminded Halsey a little bit of Miranda. Like all her AIs, Halsey implanted a viral termination code in Cortana, but she suspected, and seemed amused by, the possibility that Cortana had already discovered it and removed it.

Halsey would treat Cortana as an equal, especially in the missions regarding her, letting Cortana chose who would be her couriers. For Operation: RED FLAG, Cortana chose the Pillar of Autumn and John-117 to escort her. When her fragment had to be delivered to Aszod, Cortana chose SPARTAN-B312 of NOBLE Team to be the one to defend her. Halsey agreed with each of her choices, and wondered if her preferences were what subconsciously influenced Cortana, or if it were simply that "great minds do think alike."

Jul 'Mdama
"This human is more trouble than she is worth."

- Jul 'Mdama, remarking on Halsey's lack of apparent progress deciphering the signals between Cortana and the Guardians.

While Halsey colluded voluntarily with 'Mdama's schemes to acquire the other half of the Janus Key, she was only ever using him for his resources, hoping to claim the Librarian's "gift" for herself. She offered strategic advice, recommending the colony of Oban as a target, and encouraged Jul to drop his show of piety between the two of them, and was even trusted enough by Jul to go out into the field with a Zealot escort during the battle of Aktis IV. Nevertheless, when both the Covenant and the UNSC finally reached the Absolute Record, Halsey was quick to turn on 'Mdama, turning the Installation's Sentinels on him and his men. While Jul recovered Halsey from the Installation as they escaped its self-destruction, not realising her betrayal, the Covenant leader began to realise that was "more trouble than she is worth." During the Covenant's occupation of Kamchatka, Halsey finally took the final step in dissolving their alliance, calling in a Spartan team to extract her in exchange for crucial information on the Guardians. This resulted in 'Mdama's death before her, an event that went unremarked upon by Halsey.

Skills and abilities
At a young age Halsey was considered a prodigy. She is also a noted polymath, her expertise and knowledge making her a professional in multiple fields. She has formal degrees as a medical doctor and computer scientist, and she has demonstrated remarkable skill in AI development, quantum theory and slipspace-related phenomena, the engineering of military equipment such as power armor and related technologies, as well as xenoarchaeological research into Forerunner technology and the translation of Forerunner symbols. Because of her extensive contributions to AI development, Halsey is capable of holding her own against smart AIs, using her expertise to exploit secret programming backdoors or philosophical puzzles to control or manipulate constructs as advanced as fifth-generation smart AI. She types at a rate of 140 words per minute. Her skills as an artist, as demonstrated by the plethora of drawings in her journal, have also been noted. Catherine Halsey was also able to perfectly understand Jul 'Mdama, indicating that she has some knowledge of the Sangheili language.

According to the Librarian, a geas she planted into the human genome 100,000 years earlier encouraged the development of many of the technologies Halsey created in order to prepare humanity for a major threat they would be forced to confront as Reclaimers. The Librarian's stored essence on Requiem placed a particular responsibility on Halsey, giving her the Janus Key to advance humanity.

Attire and personal effects
Because she is most often occupied with her work, Halsey typically wears a lab coat. On Reach she was often seen wearing a skirt although she has later shown to don the standard ONI scientist utility clothes with a blue tunic and pants under her lab coat. When outdoors in Reach's Highlands and in her SWORD base archaeological site she often wore a heavy parka because of the cold. She has a set of antique bifocal glasses which feature a variety of functions, including a heads-up display, a retinal and brain pattern scanner, as well as automatic polarization. The frames contain tiny projectors which can beam classified data directly onto her retina. However, she seems to have largely abandoned her use of the glasses in the post-war era.

She typically carries a personal laptop on her as well as a purse, the latter containing a data pad, a pocket archive, a change of clothes, self-amalgamating tape, a pocket saw, a solar power pack, medications, a folding knife, her mother's antique Patek Philippe luxury watch, as well as coffee for "emergencies". Halsey had several personal AIs, including Jerrod, who could run on her laptop, and Kalmiya, who served as her assistant on Reach. Halsey had a snowglobe on her desk in her CASTLE Base office, containing a three-centimeter-tall representation of the Matterhorn being scaled by a tiny Swiss climber.

Halsey kept a journal with her ever since she was eighteen, purchased from a shop on Reach. She used the journal as a medium through which to express her thoughts and concerns, draw sketches, and map out important research notes for approximately forty-two years. It was not Halsey's only journal, since she had a second one for note-taking and also had an encrypted audio journal on her computer, though she sometimes erased its entries soon after recording them. Halsey frequently left her first journal lying around buried under papers, and procrastinated on adding some security measures to it to prevent it from being stolen. This absent-mindedness eventually backfired on her when she forgot the journal during her escape from SWORD Base. The journal was later recovered by ONI in 2553, and used as evidence to condemn her.

Production notes

 * In Halo: Reach, Halo 4, and Halo 5: Guardians, Dr. Halsey is voiced by Jen Taylor, who also voices Cortana, reinforcing the connection between the two characters. This is mirrored in Halo Legends (which predates the three games), in which Shelley Calene-Black voices both Halsey and Cortana. Natascha McElhone will play Halsey in Halo: The Television Series as well as Cortana.
 * A number of Dr. Halsey's statements are echoed by similar statements by Cortana in the Halo games. In Halo: The Fall of Reach, when John-117 explains his plan to board the Covenant ship near Chi Ceti IV, Halsey states, "One slight error in your trajectory, and you could miss by kilometers", to which John replies, "I don't miss, ma'am." A strikingly similar exchange occurs in Halo 2, with Cortana asking, "What if you miss?" and John responding, "I won't". Many of Cortana's lines during the Cortana moments in Halo 3 are direct quotes or slight paraphrases of Halsey's statements to the Spartans in Halo: The Fall of Reach. In The Package, Halsey tells John-117 that she slept well, "No thanks to their driving," which is echoed in Halo: Combat Evolved when he tells Cortana he slept well "... no thanks to [her] driving". She also tells John, "Don't make a girl a promise if you know you can't keep it," which Cortana would tell him in Halo 2.
 * In Halo Legends, Halsey's appearance was portrayed with a great deal of artistic license. In The Package, at which point she is aged fifty-two, she is depicted as a young woman with short, blonde hair and no glasses. Her appearance in Homecoming however, is closer to her description from the books, though that short also portrays her as blonde. In the developer commentary for Homecoming Frank O'Connor states, "People tend to think of her as older than she's portrayed... and part of that was simply—you know—it's anime, and they want everyone to be attractive and young. I mean, that's one of the hallmarks anime... I think that IG took the safer of the two routes and sort of disguised her appearance and disguised her age by having it literally obscured by her hair and her glasses and to a certain extent her uniform." In The Package commentary he explains, "She was definitely one of those back-and-forth conversations [with Casio Entertainment]; 'Can you make her older? Can you make her older?' And eventually we got to this compromise... I think if we were casting a real-life actress we'd cast someone a lot older, but in anime the context is fine and it works.'"
 * Halo: First Strike states that Halsey never refers to the Spartan-IIs by their numerical designations, using only their names. In Homecoming, however, she refers to Daisy-023 as "Twenty-Three" when trying to convince her to return to the SPARTAN-II program.
 * Dr. Halsey's personal journal is included in Halo: Reach ' s Limited and Legendary Editions. The journal was mostly written by Eric Nylund in conjunction with Bungie and 343 Industries, with Lorraine McLees serving as art director. Included with the journal is Halsey's ONI badge. Scanning the barcode on it with a barcode reader translates it as "HALSEY". Holding the badge under a black light reveals her face over the ONI emblem.
 * According to lead writer Brian Reed, the object of Dr. Halsey's revenge at the end of Spartan Ops was originally clear-cut — Commander Palmer and the crew of Infinity. However, this plot point was considered too blatant and out of character. To better depict Halsey's signature "double talk", this was changed to the less obvious presentation used in the final draft. During his review of Season 1, Reed suggested that Halsey's grudge may be against Commander Palmer specifically, the UNSC as a whole, or "everyone" in general. However, he implied that she was actually making a veiled threat toward Jul 'Mdama. Furthermore, Halsey was not originally planned to lose her arm in Exodus. The writers later decided to remove her arm to add a sense of ambiguity as to whom Halsey wanted revenge on.
 * Catherine Halsey is also the name of a character in the 1943 novel The Fountainhead by Ayn Rand. In the novel Halsey progresses into becoming an altruist, thinking of nothing other than the greater good of the majority. In this sense, she is an opposite of the Halsey of the Halo universe. Dr. Halsey begins by sacrificing the Spartans to buy the UNSC more time, but then changes her mind and does everything she can to save the Spartans, regardless of what happens to the rest of humanity.
 * Early Halo 5 menu concepts imply that Halsey would have been an antagonist in early drafts of the game, involving a Forerunner warship and the UNSC attempting to stop her.

List of appearances

 * Halo: The Fall of Reach
 * Adjunct
 * Halo: The Flood
 * Halo: First Strike
 * i love bees
 * Halo 2
 * Conversations from the Universe
 * Halo Graphic Novel
 * Page 122
 * Halo: Ghosts of Onyx
 * Halo 3
 * Halo: Contact Harvest
 * Halo: The Cole Protocol
 * Halo Wars: Genesis
 * Halo Wars
 * Halo 3: ODST
 * Halo Legends
 * The Babysitter
 * Homecoming
 * The Package
 * Halo: Evolutions
 * Pariah
 * Human Weakness
 * Halo: Reach
 * Data pads
 * Dr. Halsey's personal journal


 * Halo: Fall of Reach
 * Boot Camp
 * Covenant
 * Invasion
 * Halo: Glasslands
 * Halo: Combat Evolved Anniversary
 * Terminals
 * Halo: The Thursday War
 * Halo 4
 * Spartan Ops
 * Infinity Briefing Packet
 * Halo: Initiation
 * Halo: Escalation
 * Halo: Mortal Dictata
 * Halo: New Blood
 * Hunt the Truth
 * Hunt the Signal
 * Halo: Saint's Testimony
 * Halo: The Fall of Reach - The Animated Series
 * Halo 5: Guardians
 * Limited Edition
 * Halo: Ground Command
 * Halo Mythos
 * Halo Fractures
 * Lessons Learned
 * Rossbach's World


 * Halo Wars 2
 * Halo: Legacy of Onyx
 * Halo: Collateral Damage
 * Halo: Bad Blood
 * Halo: Silent Storm
 * Halo: Official Spartan Field Manual
 * Discover Hope
 * Halo: Oblivion
 * Halo Infinite
 * Halo: The Television Series