Okay, I have a theory. You know in COM we saw Zexion try to fool Riku by changing into Sora (and if you don't you're a spoiled idiot). I believe thats his ability, to change into objects and people to his advantage. I have no idea what his element might be, but maybe he doesn't have one (Xigbar didn't have one). Don't flame this, please post your (calm, please) thoughts.