Cortana is an AI. She is slender and is navy blue or lavender in color. She has short hair and a hard-angled face. When Cortana was created, the best ONI insurgency software and code-cracking skills were used. She is easily bored, because she is so intelligent, which cause Cortana to break into databases and various ONI information sources. Cortana is also built using Dr. Halsey's brain, so in a way she is a piece of or extension of Dr. Halsey. When AIs are added to the Spartan armor, Cortana chooses to work with Master Chief John. Cortana is also the AI that pilot the Pillar of Autumn.