Mackenzie: Don't Stop Believin' by Journey. She always seems so optimistic despite her "handicap".
Muffin: Bad Day by Daniel Powter? I don't know. Muffin's kind of antisocial most of the time.
Zeus: Crazy by Gnarls Barkley. Self explanitory? Except I don't think Zeus would remember when he lost his mind. It must have happened early on.