Kingdom Hearts/Secret Bosses

Sephiroth
Fought in Olympia Stadium in the Platinum Cup. Sephiroth is seen as the hardest boss in Kingdom Hearts ( original version, not Final Mix).

Enigmatic Man (Final Mix only)
The hardest boss in Final Mix. He attacks in the same way as Xemnas in Kingdom Hearts II and is found in the room in Hollow Bastion where you fought Maleficent Dragon.