Mr. Benjamin "Benny" Krupp is the alter-ego of the superhero, Captain Underpants. He is a very mean principal who hates the happiness of children. He even hates George and Harold's pranks, jokes, and smiles.
One day, after George and Harold ruined a big game, the next day, they were reported to Mr. Krupp's office and showed them their pranks on tape and wanted to find out who was responsible for the fiasco. George and Harold pleaded that they "don't want to be killed for the rest of their lives", Mr. Krupp gave them blackmail and force the two boys to do his chores and not smile. Mr. Krupp even asked their teachers to give them extra homework.
Then one day, George ordered a 3D Hypno-Ring and waited for four-to-six weeks after all the brutal punishment George and Harold suffered, they hypnotized Mr. Krupp and made him forget about the game disaster. Then George and Harold turned Mr. Krupp into Captain Underpants.
- Whenever Captain Underpants is sprayed by water, he turns back to Mr. Krupp.
- Whenever fingers are snapped, he transforms back to Captain Underpants.
- Beside being mean, he does have a soft spot for his nephew Kipper. He punished anyone who bullies him.
And he is occasionally fair to George and Harold. For example in "invasion of the Naughty cafeteria ladies from Outer Space" he refused to punish them cause the lunch ladies didn't have enough evidence to prove of their pranks. Despite changing his mind and Reprimanded them later on.
- In Captain Underpants and the Sensational Saga of Sir Stinks-a-Lot, Mr. Krupp lost his ability to turn into Captain Underpants. The book stated that there are two possible reasons for this: the electromagnetic explosion that resulted from feeding Sir Stinks-a-Lot soda, Mentos, and pop rocks had erased the hypnotic spell from Mr. Krupp's brain, or Sir Stinks-a-Lot's Super-Power-Juice-ectonomy took away Mr. Krupp's superpowers.