How do you get the one winged angel in Kingdom Hearts?

The One-Winged Angel is obtained after you defeat Sephiroth, a Final Fantasy character who is arguably the hardest foe in the game, at the Platinum Match at Olympus Coliseum. However, this only occurs in Kingdom Hearts: Final Mix, which is only available in Japanese; other than experience, there's no reward for defeating Sephiroth in the regular Kingdom Hearts.