Darkrai

Darkrai the Shadowed Moon is an intelligent, silent guardian, who works with his sister, Cresselia, to keep the moon moving around the world. Being the almost exacto opposite of his kind and compassionate sister, the only person he ever gets along even vaguely well with is Carlos, who caught him as a twelve-year-old. Darkrai is rude, violent, and also has an extreme split personality. One side is cunning, conniving, cruel, and calculating, and the other side is smarter, faster, and wilder, as well as (somehow) the opposite gender. Yes, Darkrai turns into a girl periodically. Strangely, Cresselia doesn't seem to mind...?
He is also capable of instantaneously putting his opponents to sleep, as well as causing painfully realistic nightmares, so much so that you feel the pain from within...