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 amongst enemy troops, she was recaptured by the UNSC and commissioned to work on urgent scientific matters, particularly in regards to the rise of the Guardians.

Early life and career
Catherine Elizabeth Halsey was born on the colony world Endymion, in the city of Port Vernon, on March 19, 2492. What little is known about her early life gives a glimpse of what would be a promising career. In her own words, she was "...smarter than her parents, always reading, talking, learning, and eager to share her knowledge with anyone who would listen." Halsey was sent to the elite academy known as Endymion Gifted at a young age. Continuing to defy educational standards, Halsey caught the attention of the Dean of Biological Science at Koletre-Browning University, who funded her a trip to Circumstance and continued to fund her education until 2507. By that year, at the age of 15, Catherine Halsey completed her second Doctoral Thesis paper, in which she discussed archaic line commands. She was Vice Admiral Ysionris Jeromi's star pupil as well.

In 2507, Halsey met Dr. Elias Carver at an academic social mixer, during which Carver was discussing "the matrix mechanics of the socio-and politico-economic vectors of human expansion". Halsey saw Carver's ideas as genius but his algorithm implementation as obsolete. Carver had used only seven dimensional parameters in his calculations; Halsey thought he needed sixteen and attempted to correct him. Carver did not appreciate being challenged by a young upstart. Halsey later noted that had she known that Carver would commit suicide two years later, she would have been more charitable. However, the future Vice Admiral Michael Stanforth, then a Captain, took note of Halsey. This was the start of Stanforth's continuous aiding of Dr. Halsey with her projects as her military counterpart and protecting her and her projects from those that wanted to see them disbanded in the coming decades.

Three weeks after the occasion, the UNSC approved Halsey's grant proposal for artificial intelligence control for N-dimensional matrices, under the provision that she first successfully test a model of their choosing. It was by testing this model, using both Carver's and her own dimensional parameters, that Halsey was convinced to join the UNSC and work to end the Insurrection.

In August 2510, Halsey purchased a small, hardback journal in a shop on Reach, near the base she was stationed in. She would record her thoughts and research notes in it for the next 42 years.

SPARTAN-II program


"How many lives must be spent to save all of humanity? Is any price too high to pay?"

- Dr. Halsey weighing the morality of the SPARTAN-II program.

Halsey was recruited by the Office of Naval Intelligence to work on the SPARTAN and MJOLNIR programs in 2511. Over the coming years, she devised the foundations for the next-generation supersoldier program. Reviewing the shortcomings of the previous ORION project, Halsey concluded early on that the candidates would have to be children. This enabled them to be trained and indoctrinated from a young age, ensuring their unparalleled skill and utmost loyalty to the UNSC. Additionally, the augmentations used in the program would have to be administered at the onset of puberty to reduce the risk of complications. As the augmentations were only compatible with a very narrow genetic set, Halsey built a set of rigorous genetic requirements that any potential candidate of the program was required to meet. From these requirements, she identified 150 candidates, but ONI would only give her enough funding to train 75 of them. In 2517, the project progressed to the candidate procurement stage. Time constraints made it impossible for her to personally screen every candidate from the Outer Colonies, so she delegated much of the responsibility to her team, while she processed candidates in Sector 4. Junior Lieutenant Jacob Keyes accompanied her as an attaché during the majority of these interviews, including most notably that of John-117.

Halsey then had the children secretly brought to the planet Reach for training. 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.

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.

In 2515, Halsey became a scientific advisor within ONI, becoming the youngest individual in ONI's history to hold the rank. 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.

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 her 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 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.

Return to the UNSC
"Cortana is built from a matrix of my own mind. The Domain gives her incredible power...Spartan Locke!...Stop her. But please. Bring John home to me."

- Dr. Halsey pleading Jameson Locke

In September 2558, Dr. Halsey contacted the UNSC claiming to have information on a series of recent attacks on human colonies by mysterious Forerunner entities. In response, 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. She was accompanied by Sarah Palmer, no longer in heavy animosity for her, but encountered plenty of friction from the Swords of Sanghelios, who stubbornly insisted they could take on the Guardian themselves. Regardless, Halsey devised a plan to take brief command of the Guardian, by using a Constructor Sentinel to interface with the Guardian so she could delay Cortana's control of it, lessen the damage at its point of exit, and slow it down so Osiris could board and follow the Guardian. Finding the Guardian at Sunaion, Halsey's plan worked and Osiris successfully accompanied the Guardian to Genesis. Before they left, Halsey begged Jameson Locke, leader of Osiris, to do their utmost to stop Cortana but to bring back John safely. True to their word, Osiris managed to rescue the Master Chief and Blue Team after they were imprisoned on Genesis. Halsey and Palmer returned to camp at Sanghelios with Thel 'Vadam and the Swords, where Osiris returned and landed with her old Spartans aboard with them.

Eulogy for Noble Six
"Because of you, we found Halo, unlocked its secrets, shattered our enemy's resolve. Our victory—your victory—was so close... I wish you could have lived to see it. But you belong to Reach. Your body, your armor—all burned and turned to glass. Everything...except your courage. That, you gave to us. And with it, we can rebuild."

- Dr. Halsey, commemorating Noble Six's actions that led Reach's recolonization.

On July 7, 2589, 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.

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.

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.

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 04B, 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 hald 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 UNSC 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.
 * Originally Dr. Halsey was not meant to lose her arm in Exodus. The writers later decided to remove her arm to add a sense of ambiguity as to who 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
 * 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 - Essential Tales of the Halo Universe
 * 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