Mystical stat represents the connection the character has to this outside power of 'magic'. So those that aren't naturally inclined to it, or choose to have it when they transform, have found more mundane ways to be effective. I wanted magic to feel like something that you needed to decide to take on and since characters can grow into the moves that use Mystical more, it felt natural to keep as a character choice and not something that every being on the planet can do which is roughly what all the basic moves are.