My Guess would be Shadow. He joins up with you when the mountain rises, the chaos.
When you first meat him, depending on what you do, is either with Sabin, when he got seperated from the group, or with Terra and Lock, where the End could be between Locke and Celes. You techniqely try him out in your group either way, the next time Try your luck on me being fighting him in the colloseam which you can do anytime after you save him from the cave you can challenge him anytime with the right sword, or it could mean try your luck on him by waiting at the floating continent for him.