Cortana

"“This Galaxy is vast; its wonders and beauty are almost unfathomable. But the galaxy also hides dark secrets, some of which have lain dormant since the beginning of time itself. There is a danger in secrets, both in seeking and in knowing. Some things are meant to be hidden from view. Some mysteries defy understanding, and sometimes even the things we think we know are untrue. Some secrets should remain untouched.”"

- Cortana

Cortana, UNSC Artificial intelligence (Service Number - CTN 0452-9), was a smart artificial intelligence construct. She was one of the most important figures in the Human-Covenant war, and was also John-117's partner in various combat missions as well as serving as the A.I. for the Halcyon-class light cruiser - UNSC Pillar of Autumn, Orbital Defense Platform - Cairo Station and Charon-class light frigate - UNSC Forward Unto Dawn.

In addition, she held vital data pertaining to the Halos, including the Activation Index from Installation 04.

Creation
Cortana was created using a flash cloned copy of Dr. Catherine Elizabeth Halsey's brain. Twenty flash cloned brains were created, but only two survived; one of the survivors 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 (roughly, "Quando la partita è finita, il re e pedina andare nella stessa scatola"). 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 some of 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 his 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.