Sephiroth is the main antagonist of Final Fantasy VII and the film Final Fantasy VII; Advent Children, and makes minor appearances as an optional boss in Disney and Square-Enix's game franchise, Kingdom Hearts.
Kingdom Hearts I
Sephiroth appears as an optional boss in the first Kingdom Hearts. He is found in the Olympus Colosseum, and although no backstory is really provided, the game refers to him as "The SOLDIER Commander", a reference to Shinra's SOLDIER warriors from Final Fantasy VII.
In KH: Final Mix a brief meeting between Cloud and Sephiroth can be seen, followed by a short battle. It ends with a flash of light, making the victor unclear.
Kingdom Hearts II
In the second Kingdom Hearts game, while still an optional boss, Sephiroth is given a minor storyline. Sephiroth claims he is the darkness in Cloud's heart and as long as that darkness remains Sephiroth cannot die. Thus, Cloud and Sephiroth have been fighting for a long time, across many different worlds. Cloud is seen searching in Hollow Bastion, hunting for Sephiroth, wanting to end their dispute once and for all. In the Dark Depths of Hallow Bastion (Radiant Garden), Sephiroth appears standing at the cliff where Sora was saved by Malifacient, and asks where Cloud is. Sora and company refuse to answer, then Sephiroth looks at the keyblade, and takes interest in it, wondering if it would accept a new master once he deafets Sora, thus initiating the battle. Once the player defeats him, he turns around, unscarred and in perfect condition. He then asks Sora to ask Cloud to come to him so they can finish what has begun. Once Cloud shows up, a short cutscene of their battle is shown, ending with the two disappearing through a portal.