Dr. Catherine Halsey was the driving force behind the SPARTAN-II project, as well as the creation of the MJOLNIR worn by the Spartans. She began the project in 2517 at the age of 25. She also has strong connections to other important characters in the Halo universe. She had a relationship with Jacob Keyes that resulted in a daughter named Miranda. In her journal, which came with the special edition of Halo: Reach, she explains how she urged Miranda not to join the UNSC, and this caused them to drift apart. A later journal entry says Miranda invited Dr. Halsey to her promotion ceremony on Reach, but Halsey wrote, "I didn't go. Because her father would be there? Because I am neck deep in the final stages of a half-dozen projects? I know why I didn't go. I don't want to say it." Through the novels and her journal it became obvious that, despite her perceived cold demeanor, she does care very deeply for Miranda, becoming quite emotional upon hearing of her death and realizing that she now knows how the parents of her Spartans feel as she herself has now lost a child.
The initial group of Spartan recruits consisted of 75 children, personally chosen by Dr. Halsey. Included in this group was John-117, later known as the legendary Master Chief, the greatest hero in humanity's war against the Covenant. All 75 candidates were essentially kidnapped and secretly taken to the planet Reach for training, while the families of the recruits received flash clones of their respective children. Ever since the inception of the Spartan project, Dr. Halsey has displayed an immense feeling of guilt, due to the suddenness of the project's recruitment, the rigorous training and the extreme genetic augmentations that the children were required to undergo. This process resulted in the death of over 30 recruits, which added to Halsey's uncertainty about the project. She ultimately deems the sacrifice of the children's lives necessary for the greater good of humanity, but it isn't made clear if she is ever truly at peace with her decision. It is evident that she felt they were her Spartans and she was responsible for their well being.
The project was originally called ORION when Dr. Halsey inherited it, but she felt it would be best to rename the project and separate her work from the failures of the ORION project. She decided to call her project SPARTAN because of the popularity of the Battle of Thermopylae inside the military. The project was named SPARTAN-II so as to not forget the sacrifices and the advancements that came from the original ORION project.
Dr. Halsey is also responsible for the creation of Cortana, a pivotal figure in the Human-Covenant war, and the A.I. companion of Master Chief. Cortana was constructed from a flash clone of Halsey's brain tissue. Dr. Halsey broke both civilian and military law and created clones of herself. She excised the brains from the clones and used her personality and cloned brain tissue as the AI matrix seed for the AI she wanted to give to her Spartans. She saw the AI as her gift to them. The AI she created, she named Cortana.
Dr. Halsey finalized the design of the MJOLNIR armor used by the Spartans. She based much of the design off a bulkier concept created during the ORION project. She made a number of improvements. Dr. Halsey is responsible for the addition of the shield generator, which was based off of a recovered Jackal shield. Her journal has notes for the Mark VII armor detailing that it would be powered by plasma and fusion technology and utilize atmospheric insertion, slipspace de-insertion, AI transfer protocols, and limited energy shield shaping. Spartan-II Naomi 010 was issued the Mark VII in the novel Halo: Glasslands but only described it as being lighter than the Mark VI.
She appears in the books Halo: The Fall of Reach, Halo: First Strike, Halo: Ghosts of Onyx, and Halo: Glasslands as well as the Halo Legends shorts "Homecoming" and "The Package". In the fiction, a flash clone of her brain tissue is the basis for the construction of the "smart" AI Cortana . She is viewed by the SPARTAN-IIs as a "mother" figure, and addresses each soldier by their name rather than designation, even when the spartans are fully suited in their armor. Halsey justifies her actions through her belief that the suffering of a few is acceptable for the benefit of many. Sergeant Johnson , however, unknowingly causes Halsey to rethink her position, and she decides to "save each and every member of humanity beginning with herself" during Halo: First Strike. Dr. Halsey hijacks a shuttle for her own private mission to the planet Onyx; there, she assists in deciphering the surrounding Forerunner glyphs on the planet and leads the surviving Spartan-IIs and Spartan-IIIs to a Dyson Sphere at the heart of Onyx. She, the Spartans, and Mendez were rescued by a team sent by Admiral Parangosky -- the boss of ONI with a grudge against Halsey -- whom had the doctor arrested shortly thereafter. Currently (as of March 2553) she is being detained aboard the Ivanoff, a research station orbiting Halo Installation 03.
Dr. Halsey is voiced by Jen Taylor, the same actress who voiced Cortana in all the other Halo games.