Catherine Halsey

For the SPARTAN-III, see Catherine-B320; for the Spartan Ops episode, see S1/Catherine

"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.

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 held considerable authority within ONI, despite being a civilian, and commanded respect from important military figures. Though generally well-regarded among her peers, she has personally endured intense guilt for condemning the Spartans to the life of a super soldier and has always held them in high esteem. After the Human-Covenant War, she was stripped of her position, incarcerated, and treated as a war criminal by the UNSC at large, which led to her apparent defection to the reunified Covenant.

Early life and career
Catherine Elizabeth Halsey was born on the colony world Endymion, in the city of Port Vernon, on March 19th, 2492. Not much is known about her early life, but what little is known 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." By 2507, at the age of 15, Dr. Halsey was already writing 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. Halsey used the journal as a medium through which to express her thoughts and concerns, as well as important research notes, and would do so for another forty-two years, until she abandoned it during her escape from SWORD Base. The journal was later recovered by ONI in 2553, and was designated top secret.

Dr. Halsey served as a scientific adviser for to the Office of Naval Intelligence from 2515 to 2522, when she was made chief scientist. She made huge contributions to many ONI Section III projects, including the creation and deployment of AI constructs. At some point, she supervised the creation of the template for third-generation smart AIs. She also created and initiated the SPARTAN-II project as well as the development of the MJOLNIR armor system. These all proved to be important developments for the UNSC, and to the continued survival of mankind. During her service with ONI, Halsey was stationed primarily on the planet Reach. By 2551, she lived in the community of Csongrád.

During their survey travels for the SPARTAN-II candidates, Halsey grew attached to Jacob Keyes. In November 2524 the two were reunited while attending a conference at the University of Calippus, and the two conceived a child. In 2525, Halsey gave birth to Miranda 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.

Halsey served as a mentor to Professor Ellen Anders at some point prior to 2531, though they apparently had a major falling out, as noted by Anders, saying "Why mention Halsey here? She hated me and I hated her."

SPARTAN-II Program
"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 in Elysium City.

Halsey began her work on the SPARTAN and MJOLNIR programs in 2511, improving upon the shortcomings of the ORION project. In 2517, at the age of 25, 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. 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. The Spartans held considerable respect for 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 was also privy to many of the recruits' secrets, having taught several to them as children, such as the Oly Oly Oxen Free security tune.

Battle of Harvest
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.

Next class of Spartans
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), and buried Halsey in CASTLE Base, cutting off her access to classified material. However, in just a few short years, 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.

Capture
In 2544, Halsey was captured by the Covenant's Third Fleet of Glorious Consequence during the Battle of Miridem, while she was cryonically frozen. A team of SPARTAN-IIs, consisting of John-117, Kelly-087, Frederic-104, Solomon-069, and Arthur-079 were sent to rescue her with OFA92/EVA "Booster Frame" vehicles. Knowing Halsey's value to both the UNSC and the Covenant, Fleet Master Luro 'Taralumee planted decoys in several of the fleet's CCS-class battlecruisers.

Solomon was killed by one of these decoys, which was actually an antimatter charge giving off a false signal. Arthur was also killed, crushed between two cruisers while covering Kelly-087 from multiple pursuing Seraph fighters. After finding the ship Halsey was on, John, Kelly and Fred fought through large numbers of Covenant forces, though Fred and Kelly were separated during the fight. John soon reached Halsey and rescued her from a cryo pod, 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 Thel mid-fight via gravity lift, and jumped into Slipspace.

Halsey and John entered an escape pod but were engaged by Seraph fighters. They were soon rescued by Fred and Kelly, who had captured a Seraph fighter. The surviving humans were then recovered by the stealth ship that had deployed the Spartans. Aboard the stealth ship, Halsey comforted John as he swore not to lose another Spartan.

Private projects
Over the course of the war, Halsey worked with her AIs experimenting on new ways to extend the life of smart AI and expand their processing capabilities. She also experimented on her private smart AI "triumvirate", where three AI constructs, arranged in parallel, could divide their algorithms and would decide matters by majority vote. Coincidentally, a similar arrangement was already in use by the Assembly, a group of AIs that kept its existence hidden from humans. 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 volume of Slipstream space, in an attempt to create a virtually infinite processing matrix for AIs to inhabit. Though the test was ultimately a failure, the Assembly saw the potential in the concept of residing permanently in the alternate space-time.

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 Catherine-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 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 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 prepared the package, she entrusted SPARTAN-B312 with the package: a fragment of the AI Cortana, now uploaded with the information from the Forerunner facility. Once she made certain that Noble Six would deliver Cortana to the Autumn, she left for CASTLE Base under the escort of Jun-A226 while the remaining NOBLE Team members delivered package 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 (including Cortana's "older sister" Kalmiya) 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, 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. Araqiel then proceeded to threaten her (he went as far as telling her he would fill the room with poison, and that she would be reported). She started typing in "archaic line commands" to access his code directory and personal fail-safe. She then shut him down. After shutting it down, she 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. 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, the Forerunner 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
Days later, a group of Spartans, including John-117, rescued Dr. Halsey and the remaining Spartans and evacuated them from Reach to the captured Ascendant Justice.

Dr. Halsey later saw the importance of destroying this artifact after it caused a Slipspace anomaly that nearly resulted in the destruction of the Ascendant Justice. After escaping Reach, the group fled to Eridanus Secundus in the Eridanus system's asteroid belt, where the Spartan's first mission had been in. While there, she stole the rebel governor's spacecraft Beatrice and, along with Kelly-087, whom she had sedated, escaped 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. 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; otherwise the Covenant might gain unprecedented advances to their already sophisticated slipspace technology. Soon after, the Corporal destroyed the crystal, dying 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.

Battle of Onyx
Dr. Halsey and Kelly eventually arrived at Onyx, while reuniting with SCPO Mendez and eventually her remaining SPARTAN-II's and the SPARTAN-III's. 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. 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. 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 the facility's map room, as well as allowing them to quickly move across the megastructure by using its teleportation grid.

At the end of the battle, 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 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. After the Infinity participated in the Sangheili civil war, 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. 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, 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 but 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 Spartan 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
Late in the attack, though, 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 Parangosky's order.

Halsey was then taken to the site of the Librarian, 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 hit 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, '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.

Eulogy for Noble Six
At some point after the Human-Covenant War, 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
Halsey's intelligence is unquestionable as well as the loyalty and respect she receives from the majority of her comrades and her Spartans. However, her judgment is not always correct, although bearing the guilt of the Spartan program, her sense of right and wrong has differed greatly. It is obvious that she truly cares about her Spartans, to the point she was ready to give up everything in order to save them by sheltering them in the shield world.

Halsey has been shown to be extremely devoted to her work and personal goals, and she rarely partakes in any social activities. 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. Eventually, she came to consider her AIs the closest friends she has and prefers their company. Although she may be angered easily at being mentally defeated, she always appreciates the simplicity of them. None more than Cortana, who was actually created from a flash clone of her own brain. 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 is an atheist, lacking belief in the existence of any god.

Halsey has a notable fondness for classical music, especially Debussy, and often asks nearby AIs to play select pieces as she works. 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.

Halsey 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.

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 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.

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.

Due to her burdened conscience, Halsey was consistently at odds with ONI Section Three over the course of the SPARTAN-II Program. ONI was not appreciative of hesitation or delays in what they viewed as their prize project, and constantly pressured 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 who "harbored fewer doubts" - and most likely someone less qualified as well, prompting Halsey to stay on board with the project for the sake of the candidates' safety.

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. 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.

Rivalries and public perception
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 and Fleet Admiral Hood. 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. In addition, ONI Commander-in-Chief, Admiral Margaret Parangosky, was resentful of Halsey, perceiving the doctor's aversion for ONI control and her general lack of loyalty to ONI as a threat to her own power. 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.

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. While she did indeed concoct the scientific basis for the program, Halsey was constantly 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, rather than 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. This campaign, which involved branding Halsey as a war criminal in the eyes of the public, was headed by Admiral Parangosky, who sought to place the doctor under tighter control for decades; Parangosky's policies were largely carried on to her heir, Serin Osman. Musa-096 also demonstrated a resentment for Halsey, suggesting that other former SPARTAN-IIs assigned to ONI may also have been subjected to reeducation similar to Osman. Some of the SPARTAN-IVs have also accepted this perspective on Halsey, particularly Sarah Palmer, who regards the doctor with a particular contempt; Gabriel Thorne was less quick to judge Halsey, calmly querying her about the program rather than directly 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 speculated that Halsey's ruthlessness may have been the result of a chemical imbalance.

John-117
Halsey also had somewhat of a unique relationship with John-117. Some of this may stem from him being the first subject she observed, but she had a great fondness for him and he was her favorite Spartan. Halsey told Cortana she considers him the luckiest and best of the Spartans and apparently also finds him attractive, given her embarrassed reaction to Cortana's comments. Halsey was responsible for John's first promotion to Squad Leader as a result of her acknowledging his leadership skills and suggesting it to Mendez. Halsey also had John's full trust and he always did his best to never undermine her authority.

At one point, 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 the humans and Sangheili following the Great Schism and the 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 clearly furious when she found out that the Infinity's crew had kept his return hidden from her.

Trivia

 * In Halo: Reach and Halo 4, Halsey is voiced by Jen Taylor, who also voices Cortana, reinforcing the connection between the two characters. This is also the case in Halo Legends, where Shelley Calene-Black voices both Halsey and Cortana.
 * Two of Dr. Halsey's statements in The Package are references to similar statements made in the first two Halo games. She 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.
 * Halsey's appearance changes drastically from The Package to Homecoming. In The Package, at which point she is aged fifty-two, she is inexplicably depicted as a fairly young woman with short, blond hair and no glasses. This may or may not have been reflected in the later appearance of Cortana, who was created from the replicated neural pathways of Dr. Halsey herself. Her appearance in Homecoming however, is closer to her description from the books. It should be noted that various concepts in The Package, and Halo Legends in general, result from artistic license. Her appearances in Halo: Reach and Halo 4 are much more in line with the description she received in the novels, with the exception of a lack of glasses. 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.'"
 * Although it is stated in Halo: First Strike that Halsey never referred to the SPARTAN-IIs by their numerical designations but by their names, she refers to Daisy-023 as "Twenty-Three" when trying to convince Daisy to return to the program.
 * Catherine Halsey is also the name of a character in the novel The Fountainhead. 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.
 * 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.
 * Halsey's outfit worn in The Package can be purchased for one's Xbox LIVE avatar for a price of 240 Microsoft Points.
 * The barcode on her badge reads HALSEY when scanned with a barcode reader.
 * Originally, in Exodus Halsey was not meant to lose her arm. The writers later decided to remove her arm to add a sense of ambiguity as to who Halsey wanted revenge on.

List of appearances

 * Halo: The Fall of Reach
 * Halo: The Flood
 * Halo: First Strike
 * Halo Graphic Novel
 * Page 122
 * Halo: Ghosts of Onyx
 * Halo 3
 * Halo: Contact Harvest
 * Halo: The Cole Protocol
 * Halo Wars: Genesis
 * Halo 3: ODST
 * Halo: Evolutions - Essential Tales of the Halo Universe
 * Pariah
 * Human Weakness
 * Halo Legends
 * The Babysitter
 * Homecoming
 * The Package
 * Halo: Reach
 * Halo: Glasslands
 * Halo: Combat Evolved Anniversary
 * Terminals
 * Halo: Fall of Reach
 * Boot Camp
 * Covenant
 * Invasion
 * Halo: The Thursday War
 * Halo 4
 * Spartan Ops
 * Halo: Initiation