Mr. Krupp is the mean principal of Jerome Horowitz Elementary School. 
He hates children, fun, laughter, smiles, and pranks.
He especially hates George Beard and Harold Hutchins for their " idiotic"  comics and pranks.

But.. Krupp has a side he isn't aware of, his superego... Captain Underpants! This results when you snap your fingers.
Captain Underpants is a goofy, good-hearted superhero. He has flight, super strength and is indestructible (( I think?? ))
His weakness is water which transforms him back into Mr. Krupp.