Cortana

She's naggy and bitchy but also clever and quite attractive so we'll let her off. 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.