One of my most wanted 3rd parties for Smash is PaRappa the Rapper - but a problem with him in the realm of Smash is that he's a character who openly opposes violence, as opposed to say, Isabelle, who is just a character who doesn't engage in violence. I think a good solution to make him seem more in-character would be to make him the only character who acknowledges Smash as a sport (outside of victory screen claps at least) - his eyes would never go angry, his taunts wouldn't be taunting as much as him expressing admiration at his opponent's skills, and on his victory screen he'd gives his foes words of encouragement (while still doing a victory dance of course).