Confirmed: If you had limited me to characters who actually had a chance, I'd have probably said Lyn. *sigh*. You say it can be ANYONE, though? In that case, I'd have to go with Rydia. Final Fantasy IV is my favorite RPG ever (don't you start your whining, I admit that VI is technically the better game; I've just got a soft spot for IV), and even though I love Kain the most, I concede to the fact that Rydia has way more moveset potential than him or Cecil. Three weapon proficiencies (not counting bows or unarmed), a dozen or so summons, and a full cache of black magic? Sounds like a good time to me.
Denied: Sonic. I can't imagine him having an even remotely cool moveset (any direction and button: curl into ball and strike foe!), and I'd like to leave as many third-party slots as possible open to the characters who actually deserve them (Simon Belmont, Megaman, an old Squaresoft character...surprise me).
Assist Trophy'd: I'm not using my Assist Trophy vote to oust another playable character, but rather to suggest something that would just plain make an awesome assist trophy in and of itself. Enter...the Meta-Knights! Skull guy! Flying robot fella! That jerk with the flail! And all the rest! Team up and defend the glory and honor of your fearless leader, Sir Meta Knight!