Tamamo no Mae

Though she looks like a child, she is the oldest of all of the monsters, including Alice. Because she was one of the original six ancestor monsters, she was tricked into sealing herself with the original Monster Lord. She found a way around this seal, however, and the side effect was that she was stuck in a child's body with a fraction of her original power. When she uses up her power after temporarily unsealing herself, she regresses to the mind of a child.