Modeled after the woman who recruited all the candidates for and headed up the Spartan II project, Dr. Halsey. She is the state of the art AI who was assigned to the Pillar of Autumn, the only UNSC ship to contain 2 AI, Cortana decides to leave the ship in an attempt to help John 117 (the Master Chief) in his sucide mission of capturing the hierach in charge of the covenant.
Cortana is a new design of AI that allows her to learn and adapt to the world on her own, this extra ability also leaves her with a limited capcity or lifespan of about 7 years.

Good luck to the hot little AI if you embark on the halo trilogy.