Cortana, UNSC Artificial Intelligence serial number CTN 0452-9, is a "smart" A.I. construct, or "Intelligent Agent". She was one of the most important figures in the Human-Covenant War, and served as John-117's partner in various combat missions, as well as serving as the A.I. for the United Nations Space Command Halcyon-class Cruiser Pillar of Autumn and the Orbital Defense Platform Cairo Station. In addition, she held vital data pertaining to the Halos, including the Activation Index from Installation 04.
Cortana was created using a flash cloned copy of the famous Dr. Catherine Elizabeth Halsey's brain. Twenty flash cloned brains were created, but only one survived; the survivor was used to create Cortana. The neural pathways of the cloned brain were scanned and copied through a process called Cognitive Impression Modeling. Her first words were "When the game is over, the king and pawn go into the same box" in Italian. She is one of at least two A.I.s that were created from Dr. Halsey's brain, the other being her predecessor, Kalmiya, another smart A.I.
Cortana seems to share her creator's memories, thoughts, opinions, and even values. When she was asked by Dr. Halsey to pick a "carrier", Cortana chose John-117 for their neural compatibility. She is not only a facsimile of Dr. Halsey's mind, but also a replica of her physical form, albeit at a younger age.