He's not really a ghost type. He just becomes a ghost using Trick-or-Treat, which he learned from copying his father.

And he isn't really "evil" for the sake of it. He's just taken the ghost ideology of causing mischief and messing with others to its grand extreme. The kid knows what he wants, and he won't back down until he's taken it. Not like he'll ever come remotely close to succeeding, but hey, let a boy dream. 

Also, [b]HE[/b] is going to eventually evolve into a Gardevoir, not a Gallade. Yes, that's right, a male Gardevoir. Haters gonna get their internal organs rearranged.