He used to be Principal Krupp, but now he's the crime-fighting, wedgie-dodging Captain Underpants. Make way for the only superhero wacky enough to tackle the evil Dr. Diaper. Illustrations.He used to be Principal Krupp, but now he's the crime-fighting, wedgie-dodging Captain Underpants. Make way for the only superhero wacky enough to tackle the evil Dr. Diaper. Illustrations.