Benjamin is an unbelievably hot-tempered and 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 (however, given that the two were responsible it was justified). 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, Benjamin 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, Benjamin does show forms of kindness. He will readily punish anyone who is bullying his nephew Kipper Krupp. 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). Benjamin always believes his nephew's claims no matter how illogical they seem.
Benjamin 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 Benjamin 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 would never get hypnotized by George and Harold and become Captain Underpants, and the villains from the first three books would take over the world, end up in a rough battle which eventually resulted in the world’s destruction.
Depiction in the Film
Krupp is portrayed in the same way as his book self. However, in the movie, he was given some sympathetic qualities where he had very little in the original books. He has a crush on Edith the Lunch Lady (who, as well, has a crush on him).
- Benjamin's nickname, Benny Krupp, is a pun on the word "bankrupt", and the name Mr. Krupp is a pun on "interrupt", "corrupt" and "disrupt".
- As revealed in Captain Underpants and the Wrath of the Wicked Wedgie Woman, Benjamin is Jewish, and most likely a Reform Jew.
- In the first book, he hates the Captain Underpants Comic Books that George and Harold make. Ironically, he would later 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, Benjamin 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.
- Dav revealed that Benjamin's birthday is April 1st, which is April Fools Day (same birthday as Professor Poopypants) and that he is in his mid-forties.
- In the film, he was given a love interest, who is Edith the Lunch Lady.
- In the film, the gold plate on the front of his desk was changed from 'Kneel Here' to 'Hope Dies Here'. Even though, some of the early film promotional pictures had 'Kneel Here' on it (or the plate was blank).
- 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.
- It is revealed that Krupp is lonely at heart and lives in an empty home with no one to care about him. His mattress has an imprint of his body where he normally sleeps, he has only 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 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 (unknown by Krupp) help him and Edith get together on a date.
- He himself has (accidentally) snapped himself into becoming Captain Underpants twice. One time in book 5 and the second time in the film at the end. Mr. Krupp also snaps himself turning into Captain Underpants in Mr. Krupp only redeems and reformed himself later in the film, but never redeems himself in the book series. In an interview, Dav Pilkey said he was happy with the film giving Krupp actual character development and a backstory; to the point where Dav wished he did the same in the books.
- It is possible that this is the reason why he had Petey reform in the Dog Man series.
- Mr. Krupp had a habit of making up rules and scribbling them in the rulebook, which got him fired in the show. When Melvinborg expelled George and Harold for breaking the rules in that same book, he got struck by bureaucracy, as the rules that were broken were unofficial, and therefore they did nothing wrong.
- In Captain Underpants: The First Epic Movie, Mr. Krupp's tie is brown, and he wears green pants, also, his white shirt has a pocket, in The Epic Tales of Captain Underpants, he wears a purple tie, and brown pants, and there's no pocket on his white shirt.
- The Captain Underpants in the books seems to be a lot smarter than the Captain Underpants in the TV show