Given how frequently AIs turn out to be villains, you're forgiven for being suspicious of Cortana. Halo: Combat Evolved toyed with your suspicion, portraying Cortana as hungry for knowledge and so powerful she barely needed you at all. But, at least in the first game, that was all a trick – Cortana was trustworthy after all, and the only one smart enough to figure who the real villain was. Thanks, Cortana.
Iconic moment: As Master Chief slots Cortana into his armor, she notes its architecture isn't that different from the warship she normally controls. "Don't get any funny ideas," the Chief quips.