For those not in the know, Cortana is well past the age an AI should be, and she now suffers from Rampancy, which has essentially turned her evil.

Is Cortana a villain?

She's back in Halo 5: Guardians, out today. But Cortana isn't your friend in Halo 5. She's the villain. Cortana, as it turned out, was the main character of Halo 4 and now, the main character of Halo 5 as well.

Is Cortana the villain in Halo Infinite?

Initially appearing as the deuteragonist of both the original Halo trilogy and Halo 4, she returns as the main antagonist of the Reclaimer Saga, serving as the main antagonist of Halo 5: Guardians, the unseen overarching antagonist of Halo Wars 2 and the posthumous overarching antagonist of Halo Infinite.