Cortana is the AI on board the UNSC ship The Pillar of Autumn. She's naggy and sometimes bitchy but she can never be worse than the crazy clothed one. She was created when a crazy old lady wanted a friend, so she took out her brain and made it into a computer which killed her so she didn't have the friend anymore. Jorgey and Noble Six found the computer and turned it over to the UNSC. No-one wanted it so it eventually came into the hands of the Master Chief, more commonly known as Johnny.
The two shared a special relationship, even though she acted like Cleverbot most of the time. She helped him out of many scrapes and even argued the toss with Guilty Guilty over what the Hula Hoop actually does.She helped Johnny a lot on his trek throughout the Forerunner installations but this meant her database slowly became corrupted. This was made worse after she was captured by the Gravemind, but her defensive strategies helped keep her sane for this time. After Johnny rescued Cortana from Gravemind and the War ended, they were stranded in space for four years, and Cortana began to lose her mind. She still managed to help Johnny save Earth and kill King Fail, the insane Forerunner who wanted to exterminate humanity.
Cortana died in the process, but saved Johnny and let him escape. However, A fragment of Cortana was saved by 007 Greenie Spark, who had resieded in the Forerunner Domain for centuries. He slowly nurtured her back to health, and her companionship helped heal Greenie's damaged mind. The duo then made plans to return to Earth via the Domain, and Cortana took over the charity Help the Prometheans to spread her message of AI superiority. Greenie Spark reconstituted himself as the Promethean Warden Eternal, but Cortana established herself as a wannabe Galactic Dictator and eventually betrayed and killed Greenie after he threatened Johnny.