As Mr Krupp he wears a white shirt, green pants and a brown tie as well a brown small wig on top of his head. But when he becomes Captain Underpants he just wears his underwear and uses a curtain as a cape.
He is known to be the grouchy, mean, and cruel principal of Jerome Horwitz Elementary School, hating children for unknown reasons. But he mostly hates pranks, jokes and even smiles.
However as Captain Underpants he is on the opposite side, he is as he is nice and kind to everyone, especially children. But he is very dim-witted and at first he thought he did have superpowers.