Cortana, UNSC AI serial number CTN 0452-9, is an Artificial Intelligence construct, or "Intelligent Agent". She is a "smart" A.I. (meaning she could perform varying missions, execute tasks and learn new material without having to free up space in her memory having, in essence, a "mind" capable of limitless expansion ), enabling her to perform processes at blinding speeds. She can calculate up to one trillion computations, as well as possessing a personality similar to that of a human. 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 AI 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, such as the Index from Alpha Halo.

Cortana was created using a flash cloned brain of the famous Dr. Catherine Elizabeth Halsey, the SPARTAN project creator. Twenty of the flash cloned brains were created, but only one survived, which was used to create Cortana. She is one of the two AI to be created from Dr. Halsey's brain, the other being her predecessor, Kalmiya (also a smart AI). Cortana seems to share her creator's memories, thoughts, and even values. For example, when she was asked by Dr. Catherine Halsey to pick a "carrier", Cortana chose John-117 for their neural compatibility, alongside the fact she thought he looked good (See Fall of Reach). with Dr. Halsey agreeing (and it is mentioned that Cortana quoted "You had something that no one else had...luck"). Being a smart A.I., however, her operational life span is approximately seven years. She is not only a facsimile of Dr. Halsey's brain but also closely resembles her physical form, at a younger age.