Sephiroth is the embodiment of Cloud's dark side. He is regarded as one of the most powerful villains in the Kingdom Hearts series, changing from an optional boss to a minor story character from Kingdom Hearts to Kingdom Hearts II.

He, along with Cloud, Tifa, Aerith, Yuffie, Zack, and Cid, come originally from the Square Enix game Final Fantasy VII, where he served as the main antagonist.