His costume consists of underpants and a red cape.
Captain Underpants is a comic book character who was made up by George Beard and Harold Hutchins, two mischievous, yet kind hearted boys to attended Jerome Horowitz Elementary School, a school which discouraged creativity.
Once, Mr. Krupp, the sadistic and grouchy principal, caught George and Harold on a video proving that they ruined the Hortwiz Knuckleheads football Game. As a punishment, he sadistically blackmailed them by threatening to show the tape to their parents and the angry football team unless they completely behaved themselves, received lots of homework and spent all their free time doing chores for him like washing his car, clipping his finger and toenails, and vacuuming his house, essentially using them for child labor.
George and Harold had got sick of being abused and bought a 3D-Hypno Ring and hypnotized Mr. Krupp, deciding to have some fun for once. They hypnotized him into thinking he was Captain Underpants. They found it funny, until he ran off and actually tried to fight crime.
Now, whenever Mr. Krupp hears someone snap their fingers, he takes off his toupee and his clothes, ties a red polka-dotted curtain around his neck and essentially turns into Captain Underpants. Also, whenever "Captain Underpants" gets his head wet, he transforms back into his usual, grumpy self.