Mario & Sonic Guy
Old rivalries live on!
We don't really know if he is the main antagonist or not. He might just be a supportive antagonist for someone more sinister. Obviously, Ganondorf isn't in the story, since it takes place before he was even born.Ghirahim is an upcoming main antagonist of The Legend of Zelda: Skyward Swords. He's the only newcomers that wouldn't end up sucking or be a clone.
Anyway, an Assist Trophy may work, but I don't see Ghirahim as being playable at all.