The Skull Kid is very mischievous and much like the Happy Mask Salesman, he has a slightly higher tone to his voice. He almost sounds insane. I'd imagine him as a nonhuman, almost deity like being. I would not got high pitched though, as I feel this character has more of a grip on his once human side than others. I would go for a decent amount passed medium pitch.  Think of him as a mischievous and trickster child.