8" tall Cortana is a super-intelligent Artificial Intelligence (AI) - a sentient, self-aware computer construct. Cortana appears in physical form as a hologram, in the self-determined guise of a human female.

Once the shipboard AI of the human starship, the Pillar of Autumn, Cortana accompanies the Master Chief on missions, "piggybacked" in his specialized MJOLNIR Mark V Assault Armor. In this role, she can provide him with intelligence data, navigation, translations, and advice.

Her genius intellect is accompanied by a wry wit and deep concern for the welfare of her human associates. With almost limitless computer intrusion and manipulation abilities, Cortana may be the most effective weapon in the Master Chief's arsenal.