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.

Catherine Elizabeth Halsey, MD, PhD, (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 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. At a young age, she 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 quote of Benjamin Franklin's for her own ends. After arriving at Reach 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 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 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. 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.

By June 19, 2513, Doctor Halsey had 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 Déjà and Jorjet 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 suggsted 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 17 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, 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 17, 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. 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 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-operation 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. By June 5, 2522 Halsey had finally been 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. 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.

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.

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 ben 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. 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.

ONI career
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. However, following the onset of the Human-Covenant War, her Spartans would prove themselves to be the most effective weapon the UNSC had against the Covenant. 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.

After seven years spent as a scientific advisor, Halsey was made ONI's chief scientist on June 5, 2522. 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. For these AIs, she created a new series of intrusion routines known as the Illegal Entry Protocols and Counter-Illegal Entry Protocols, later nicknamed, (much to Halsey's chagrin), "PIEs" and "C-PIEs", respectively, after misspellings of their acronyms. These protocols were created in response to the UNSC's standardization of their computer systems which had left them vulnerable to attack by the conversely diverse insurgent software. As an unplanned side effect, the protocols also enabled the AIs to defend UNSC operating systems from attack. Later, during the Human-Covenant War, the protocols were used to reverse-engineer Covenant technology, namely the Jackal shield gauntlet. These all proved to be important developments for the UNSC, and to the continued survival of mankind. During her career, she often clashed with Dr. Forester in regard to her theories on Slipstream space and AIs.

Throughout the decades, Halsey conducted significant research into new ways to extend the life of smart AI and expand their processing capabilities. She privately experimented on a smart AI "triumvirate", a set of three AI constructs arranged in parallel, which could divide their algorithms and decide matters by majority vote. In 2547, after thoroughly researching the subject, she stole the decommissioned Slipspace drive of the, and conducted an experiment on building an abstract fractal in the eleven-dimensional spacetime of slipspace, in an attempt to create a virtually infinite processing matrix for AIs to inhabit. Though the test was ultimately a failure, the AI collective known as the Assembly saw the potential in the concept of residing permanently in the alternate space-time. Halsey also 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.

After first contact with the Covenant, Dr. Halsey accessed LCDR al-Cygni's ONI sloop Walk of Shame's data archive. During Dr. Halsey's investigation, she began researching the last conversations of the AI Mack, and his choice of misspelled Shakespeare during his last days of rampancy.

Capture and rescue
In 2544, Halsey was present on the colony world of Miridem when the Covenant attacked the planet. Spartan Sheila-065 was killed while securing Halsey's escape. However, her evacuation transport was seized by the Covenant after she had already been cryonically frozen and she was taken aboard Resplendent Fervor, the flagship of the Covenant fleet. A team of Spartan-IIs, consisting of John-117, Kelly-087, Fred-104, Solomon-069, and Arthur-079 were sent to rescue her with OF92 Booster Frames. Knowing Halsey's value to both the UNSC and the Covenant, Fleet Master Luro 'Taralumee planted decoys in several of the fleet's battlecruisers.

While Solomon and Arthur were killed on the way, the remaining Spartans eventually found their way aboard Resplendent Fervor. The Spartans fought through large numbers of Covenant forces, though Fred and Kelly were separated during the fight. John soon reached Halsey and rescued her from her cryotube, but the two were immediately stopped by Major Thel 'Lodamee. Halsey watched as John engaged the Elite in an energy sword duel. However, Fleet Master 'Taralumee detached the section of the ship they were in, retrieving 'Lodamee mid-fight via gravity lift, and jumped into Slipspace. Halsey and John entered an escape pod and were soon rescued, along with Fred and Kelly, by the stealth ship that had deployed the Spartans. Aboard the stealth ship, Halsey comforted John as he swore not to lose another Spartan.

Slipspace and A.I. research
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.

Fall of Reach
"We knew this day would come. They have found our fortress among the stars. The Covenant are on Reach. They will burn this planet, kill millions, and when Reach falls—and fall it will—there will be nothing left to stand between them and Earth. Yet, even in our darkest hour, hope remains. Now, who will protect it? Show me NOBLE Team."

- Catherine Halsey during her discussion with Cortana about the plans for her safe delivery.



In 2552, Halsey was researching various Forerunner discoveries made on Reach, including an excavation for an underground Forerunner facility beneath ONI's SWORD Base and artifacts discovered in Visegrád by Professor Laszlo Sorvad, a civilian xenoarchaeologist in the employ of ONI, although her permanent private office remained in CASTLE Base.

During the opening stages of the Fall of Reach on July 26, 2552, Halsey was working on the excavation site beneath SWORD Base, when the facility came under attack by a Covenant corvette. After the destruction of the corvette, she called NOBLE Team to her laboratory for debriefing, where she learned of the death of Professor Sorvad at the hands of a Sangheili Zealot survey team. After arguing with Carter-A259 over Noble's failure to apprehend the Elites' leader and Kat-B320's attempted access of restricted files, she learned of the nature of Sorvad's "latchkey" discovery, a way to decipher data found in the Forerunner complex beneath Sword Base. After dismissing NOBLE Team, she began the decryption process of the "latchkey" data in the Forerunner complex.

On August 3, Halsey split up Cortana, assigning her "copy" to work on the decryption of the Forerunner data in the underground facility below Sword Base, while her other half would prepare for the Spartans' mission; Operation: RED FLAG. When the arrived at Reach from Sigma Octanus on August 12, Halsey visited the ship. During her visit, Captain Keyes came to see her, and they discussed their daughter Miranda, the Keyes Loop, the Spartans and Operation RED FLAG. Later in the day, she visited Camp Hathcock to see John-117 after his debriefing. She was evidently working underneath SWORD Base when the facility was bombarded by the Covenant on August 14, although she survived the assault and later checked in her office in CASTLE Base to discuss the specifics of RED FLAG with Cortana on August 25. Two days later, she met most of the remaining Spartan-IIs at the Reach FLEETCOM Military Complex in the briefing for RED FLAG.

On August 29, she was present in the first test of "smart" AI integration with MJOLNIR Mark V armor. Although John-117 and Cortana were nearly killed as a result of Colonel Ackerson's involvement, the test was ultimately successful. After the test, Halsey sent an official protest, backed by Vice Admiral Michael Stanforth, to, suggesting that Ackerson may have suffered a mental breakdown and recommended that he be taken into custody. Later that day, Halsey returned to her work in the SWORD Base excavation and had NOBLE Team sent to SWORD Base with a pretext of orders to destroy the facility.

Once NOBLE Team had arrived at SWORD, Halsey revealed Noble's true orders to them: to transport a package containing "mankind's best chance for survival" to the Halcyon-class cruiser Pillar of Autumn, holding position in the Aszod ship breaking yards. After Noble's successful defense of her lab while she made the final preparations, she entrusted SPARTAN-B312 with the package: a fragment of the AI Cortana now uploaded with the information from the Forerunner facility, secured in an armored matrix. Once she made certain that Noble Six would deliver Cortana to the Autumn, she left for CASTLE Base under the escort of Jun-A266 while the remaining NOBLE Team members delivered Cortana's fragment to the Autumn.

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.

Unbeknown 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. She helped decipher the mystery of the planet. 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.

Return to Infinity
"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.

Halsey was later returned aboard the Infinity during the ship's second expedition to the Forerunner shield world Requiem in 2558 as her unique expertise was required to study a mysterious Forerunner artifact. Parangosky 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. Upon arriving on the Infinity aboard the prowler, she demanded she be uncuffed before examining the artifact. 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 start of October Doctor Halsey managed to contact 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.

Around three weeks after Halsey contacted Infinity the UNSC deployed Spartan-IV Fireteam Osiris to retrieve Halsey from the custody of Jul 'Mdama on Kamchatka. Osiris arrived, killed 'Mdama, and rescued Halsey, bringing her back to the UNSC Infinity alive. Halsey was distressed to see that the UNSC took so long to retrieve her, having dire news for them: Cortana was alive, active in the Domain network, and was responsible for the awakening of the Guardians. After a following Guardian incident on Meridian, Halsey traveled to Sanghelios with Infinity's crew in anticipation of another Guardian awakening there.

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 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. 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 Epicetus 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 before she'd absconded from Eridanus Secundus with Kelly and the Beatrice. 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.

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.

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.

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
With an IQ of roughly 200, Halsey is one of the most intelligent humans alive in the 26th century. 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.
 * 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: The Television Series