Sephiroth was a character who first appeared in the video game Final Fantasy VII, however in this continuity he appears in Kingdom Hearts: Birth by Pain where he serves as one of the primary antagonists. He also has appeared in multiple Kingdom Hearts games set afterward. He was once a great hero, but would become corrupted by Chernabog, the Black God.
Birth by Pain
He started out as a great hero for the Radiant Garden, but eventually became the apprentice and disciple to Chernabog, who was currently under the alias of Gobanrehc. However Sephiroth was his one ally that he revealed his true identity to. He taught him in the ways of darkness and put him on a mission to help further his plans.
He in his original form serves as the final boss of Nexi's Scenario in the game, however he returns for another boss fight in his one winged angel form in the Final Scenario.
The Ultimate Gathering