Cortana (Earth-2915)

"I know what you're thinking, and it's crazy. Unfortunately for us both ... I like crazy."

- Cortana to John-117

Cortana (serial number: CTN 0452-9) was an Auton synthetic in service with the United Nations Space Command. Unique and exceptionally intelligent even among Autons, she was one of the most important figures in the Human-Covenant War, and served as John-117's partner in the final months of the conflict. Her flexibility and tactical skill proved invaluable in boosting the combat ability of the Halcyon-class light cruiser Pillar of Autumn, the orbital defense platform Cairo Station and the Stalwart-class light frigate In Amber Clad. In addition, she held vital data pertaining to the Halo Array, including the activation index from Installation 04.

Source

 * https://www.halopedia.org/Cortana