Doctor Catherine Halsey was born on the human colony of Endymion, where she was noted for her intellect, attending the prestigious Endymion Gifted academy. She became ONI science adviser in 2515 at the age of 23, and in 2522 became ONI's chief scientist. Soon after she began the Spartan-II program, where she kidnapped children with gifted genetics and turned them into super-soldiers. She had a continual role throughout the war with the Covenant, continually advancing Project MJOLNIR, as well as performing various other experiments, including creating the smart AI Cortana from a brain of one of her own clones.
Halsey survived the war, and after discovering Forerunner technology on Shield World 006, helped to integrate that technology with UNSC warships and equipment. Her celebrity within ONI quickly waned after the war, however, after many did not accept the moral and ethical violations Halsey perpetrated in creating the Spartan-IIs. She proved to be too valuable to be killed however, and continued to assist the UNSC, and later Jul 'Mdama's Covenant in discovering more Forerunner technology.