The AI that becomes Chell's ally in Portal 2 is Wheatley. He is a personality core created by Aperture Science and voiced by British comedian and actor Stephen Merchant.
Wheatley awakens Chell from long-term stasis and initially helps her navigate the deteriorating Aperture facility. He finds the portal gun and guides her through maintenance areas while she attempts to escape. His actions also accidentally reactivate GLaDOS, restarting the central conflict of the game.
Wheatley is often described simply as an AI, but the game more specifically presents him as a mobile personality core, or Intelligence Dampening Sphere. His nervous, fast-talking personality contrasts with GLaDOS's colder and more controlled manner. Although he begins as an ally, his later access to the facility's central computer system causes his behavior and ambitions to change dramatically.