Cortana is a smart AI developed by Doctor Catherine Halsey on Reach from a clone of her own brain in 2549. She served as the shipboard AI on the UNSC Pillar of Autumn during and following the Fall of Reach, though is best known as the AI that served with John-117 during the final battles of the Human-Covenant War. Due to her vast intelligence, Cortana was able to help John defeat the Flood at Installation 04, assisted in the Battle for Earth, and fought a battle of wits with the Gravemind aboard High Charity during the Battle of Installation 05. After she and John were lost aboard the UNSC Forward Unto Dawn following the Battle of Installation 00, she remained alone for over four years while the ship drifted and John remained in cryo-sleep.
Upon John's revival above Requiem, Cortana assisted him with fighting the Ur-Didact, despite feeling the effects of rampancy, and sacrificed herself to protect humanity and John. Part of her survived, however, and ended up on Genesis, where she came into contact with the Domain and gained access to much of the Forerunner technology in the galaxy.
While her motivations are unclear, Cortana used the power of the Domain to take control of many Forerunner sites, subjugating the known galaxy under her rule, and beginning the Rise of the Created. Though it is not proven, it is thought that Cortana's sudden change in allegiances against the UNSC was in part due to her rampancy, coupled with her exposure to the Gravemind.