I got something here: a pokemon that is based from a spilt personality, like it'd be a Dark/Fairy, to represent that completely side of a personality. It'd end up being humanoid I think and the design would reflect it by having an obvious split, either simply down the middle or have multiple split areas. As for abilites, it'd probably end up being something like what Aegislash has in Stance Change, again to represent the different personalities.
It may be a bit of a long shot for an idea but I'm actually slightly proud of it.