Principal Benjamin "Benny" Krupp is the principal of Jerome Horwitz Elementary School. He has a long and bitter history with two fourth grade students named George and Harold due to their constant pranks,
wisecracks, and comic books. Due to the effects of George's 3-D Hypno-Ring, Mr. Krupp's alter ego is Captain Underpants, which he is totally unaware of.
He and his alter ego are voiced by Ed Helms in the film Captain Underpants: The First Epic Movie.
Mr. Krupp is an unbelievably nasty principal. He distributes detention slips to any student for even the most minor infraction, just so long as they are unfortunate enough to cross his path. In one case, he punished George and Harold by not allowing them to have cafeteria food, and makes them bring their own lunches in his office to keep an eye on them, even though he had no actual proof. He once blackmailed George and Harold into doing all his daywork and even giving them extra homework in exchange for not getting "killed every day for the rest of their lives". He even tore out the last seven pages of the book, "How The Grinch Stole Christmas", to make the plot sound more realistic, assumingly cutting the book off right before the Grinch's heart grows.
His hatred towards George and Harold reaches the point where he tries to find a way to get rid of them (even if it meant that they would be killed) or (to a lesser extent) separate them somehow. In the ninth book, he was greatly happy that Tippy Tinkletrousers was going to destroy the duo, and willingly told the Turbo Toilet 2000 where George and Harold hang out in the eleventh book, so they would be gone. Also when George and Harold flunked their "Super Secret" tests, Mr. Krupp and the faculty were greatly overjoyed that Harold flunked the fourth grade because he and George would no longer be in the same grade and thus have completely different paths.
Despite being really nasty, Mr. Krupp does show forms of kindness. He will readily punish anyone who is bullying his nephew Kipper. Kipper just needs to yell out for his uncle and explain to him what happened, though Kipper usually makes it look like he was bullied in front of his uncle (For example: after Kipper punched a little girl in the stomach, Kipper told his uncle that the girl's stomach hit his first). Mr. Krupp always believes his nephew's claims no matter how illogical they seem.
Mr. Krupp also cares about his job, as he feared he would lose his job in the second book after seeing the school destroyed by the Talking Toilets until George and Harold managed to fix the school. Additionally, when Tippy Tinkletrousers unintentionally changed the past, he unintentionally caused Mr. Krupp to be fired from his post as principal. Mr. Krupp was shown shedding a tear as he was walking away with his "Krupp Stuff." With that happening also meant that in a few more years, he himself never became hypnotized to become Captain Underpants and the villains from the first three books take over.
- Mr. Krupp's nickname, Benny Krupp, is a pun on the word "bankrupt", and the name Mr. Krupp is an pun on "interrupt", "corrupt" and "disrupt".
- As revealed in Captain Underpants and the Wrath of the Wicked Wedgie Woman, Mr. Krupp is Jewish.
- In the first book, he hates the Captain Underpants Comic Books George and Harold makes. Ironically, he would become Captain Underpants himself.
- Even though he is unaware that he is Captain Underpants, there are several times where he is doused with water in the middle of the battle with an enemy. When this happens, Krupp is angry and confused as to what is happening, where he is, and why is he wearing his own curtains around his neck and in his underwear in public.
- In the film, while George and Harold bring him home as he is knocked out on a wagon, they start to plunder in his home but found nothing interesting. On the outside, signs say "keep out", "Danger" or "Go away". No one lives in the house but him.
- His mattress has a print of his body where he normally sleeps he only has one fork in the drawer, a towel with the words "His" and the other towel "Still His", the dining table has only one plate and a glass placed there.
- Towards the end of the film, he realizes that George and Harold's Captain Underpants comics are humorous and funny and he had a slight change of heart about the boys since they help him and Edith get together on a date.
- He himself has (accidentally) snapped himself into becoming Captain Underpants twice. Once time in book 5 and the second time in the film at the end.