A Woman of Many Talents
She was always a smart child, she was "smarter than her parents, always reading, talking, learning, and eager to share her knowledge with anyone who would listen." Only 15 years old, Catherine had already finished one PhD and was already writing her second Doctoral Thesis paper. ONI recruited her in 2510 as a civilian consultant and brought her to Reach. While stationed on Reach, Catherine bought the journal which she would use as a medium through which to express her thoughts and concerns, as well as pen important research notes, and would use for another forty-two years. Her research involved both dumb and smart AI (a continuation of her earlier work), and later the Mjolnir Powered Assault Armor.
For a more detailed story, please refer to Halo101: SPARTANs.
While working for ONI, Catherine found out about the ORION Project. Immediately interested, she planned to improve the project as she thought those researchers were "amateurs led by uninformed career military". This successor project would be designated the ORION Project Generation II which would later be known as the SPARTAN-II Program. She chose the name "SPARTAN" as a reminder to ancient Greece, as she meant to train "her" SPARTANs the same way. She did not want to use consenting adults as subjects, but instead used children which would be raised into their warrior mentality. These children were to be selected by finding children of the age of six which had the "perfect" combination of genes. When these children were found, they would be replaced by flash clones and they themselves would be put into the SPARTAN-II Program.
During their survey travels for the SPARTAN-II candidates, Catherine grew attached to Jacob Keyes. The two were involved in a romantic relationship in the early 2520s, and in 2525, Halsey gave birth to Miranda and raised her for a few years, when on 12 July 2531, Jacob 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.
Many, including herself, questioned whether or not her use of children was justified. Not only were the children taken away from their family, but they were given no choice in the matter, and could not return to their previous life. This guilt led to Catherine asking a would-be SPARTAN-II whether or not he wanted to join her "experiment" or stay on in the foster care where she got him. Prior to the SPARTAN-II augmentation procedures, Catherine once again asked Soren whether or not he would like to participate. Confused, Soren asked her why he was the one selected to make the choice, to which Catherine replied, it simply turned out that way. Even then, when Soren was completely aware of the risks, he choose to go through with it. This led Catherine develop the believe that all children felt this way, and she was content with this idea.
The Great War
The SPARTANs she had created were meant against the Insurrectionists before the Covenant attacked the planet Harvest to start the Human-Covenant war. This war raged for many years, and did not leave Catherine unscathed.
When Catherine was on the planet Miridem in 2544 the Covenant attacked the planet, Catherine boarded one of many evacuation crafts where she was put into cryosleep. This ship was captured however, and Catherine was (while in stasis) a prisoner of the Covenant, later transported to the Third Fleet of Glorious Consequence. Here she was rescued by a team of SPARTANs led by John-117, costing the life of Petty Officer Solomon-069 and Arthur-079.
8 years later, after the Fall of Reach, she was once again in the middle of the fighting. Having stolen a vessel to transport herself and a (kidnapped) critically wounded Kelly-087 to Onyx. Where she would be reunited with remaining SPARTAN-IIs and SPARTAN-IIIs. At first, she tried to maintain secrecy about her intentions out of habit, until Kurt-051 confronted her about it. She admitted that she was not actually looking for 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 the 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. At the end of the battle, she, along with Franklin Mendez, Blue Team, and the SPARTAN-III survivors, found herself inside the Shield World, which she believed was actually a Micro Dyson Sphere. Here, they were stuck, and needed to be rescued in order to escape, after managing to send a message to ONI, they sent Kilo-Five. However, Kilo-Five was sent in to arrest Halsey for the crimes she had committed (hijacking ships and kidnapping a SPARTAN). After her arrest, she was sent to the UNSC Infinity to work on upgrading the war ship.
Halsey was later interrogated by an unknown individual regarding the SPARTAN-II Program. She defended the justifications for creating her SPARTANs, reminding her interrogator that they saved humanity. The interrogator then meticulously noted that the SPARTAN-IIs developed sociopathic tendencies. Halsey realized that her interrogator was looking for a reason to replace John-117, and insisted that the best of her SPARTANs was still alive and should not be underestimated.
Continuing her work on the Infinity, she was put on the task of identifying the artefacts that had been recovered on Requiem. This led her to a device which led her to keep in contact with the leader of Jul 'Mdama's Covenant Jul 'Mdama. When this was found out by SPARTAN-IV Palmer they arrested Halsey and questioned her. She defended that she did not give any information to her contact. She responded by using an AI override protocol on Roland to escape and gather what information the UNSC had on the Librarian, all of which was her own work, save for a debriefing of John-117, whom she apparently believed dead, after the New Phoenix Incident. She then got into communication with 'Mdama, offering her assistance to find the Librarian. However, Roland then managed to overcome the override and cut the communication and summoned security. Captain Lasky then confronted Halsey and planned to place her in cryo, but she slaps him in the face, accusing the Captain of withholding the truth about John-117's survival.
But before Catherine can be put into cryo, she was seized by a Promethean Knight and teleported away. The new head of ONI, Serin Osman, then to have Catherine eliminated should she ever be found again. Catherine had been teleported to Jul 'Mdama, who was in possession of several Forerunner artefacts, some of which were imprinted with the Librarian. The gift Halsey received was called the Janus Key, an artefact which showed every piece of Forerunner technology in the galaxy, this included all the Halo Installation locations as well as hidden Forerunner technology, that could propel Humanity into its next stage of technological supremacy. However, when Halsey accepted the gift she was launched back into Jul 'Mdama's clutches where he relieved Halsey of one piece of the Key, not realizing she had the second.
When SPARTAN-IV Gabriel Thorne had gained entry into the Shrine and assassinated one of 'Mdama's bodyguards, Halsey quickly threw the second piece of the Key to Thorne just as Sarah Palmer arrives. Startled, 'Mdama summons the Promethean Knights to protect him, where Palmer attempts to assassinate Halsey but only managed a shot at her shoulder, 'Mdama soon escapes with the help of a Promethean Knight with Halsey as his hostage. An enraged Commander Palmer demands to know what Majestic was doing there, and when told their mission she questions why they would save a traitor, to which Thorne attempts to change her opinion by stating why had Halsey thrown him the Key if she was working with 'Mdama. After Requiem is destroyed, Halsey is seen aboard 'Mdama's ship with her left arm amputated. Believing that both Palmer and Fireteam Majestic were sent to assassinate her, she offers to assist Jul 'Mdama in exchange for revenge against the UNSC.