Principal Krupp

Mr. Krupp is a mostly bald, overweight man with a pear-shaped body and a piggish nose. As shown by the design of a dummy that resembles him, he has green eyes. Sometimes, when he laughs or becomes angry, his teeth grow sharp.

Benjamin covers his bald head with a messy, spider-like toupee. He wears an office shirt, a tie, and a pair of trousers.

In the book series, his clothes change color depending on the story. In the movie, Benjamin's tie is brown, and he wears green pants. Also, his white shirt has a pocket. In the Netflix series, he wears a purple tie and brown pants. There is also no pocket on his white shirt.

Benjamin Krupp is an unbelievably arrogant, unsympathetic, rude, impolite, grumpy, egotistical principal. He distributes detention slips to any student for even a minor infraction, just so long as they are unfortunate enough to cross his path. In one case, Benny 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). Benjamin 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 tore out the last seven pages of the book, "How The Grinch Stole Christmas", and to make the plot sound more realistic, assumingly cutting the book off right before the Grinch's heart grows.

Mr. Krupp 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 Professor Poopypants/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 unlikeable, Mr. Krupp 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). Mr. Krupp always believes his nephew's claims, no matter how illogical they seem.

When George saved Harold from Kipper and his cronies, the two became best friends. Unfortunately, they weren't aware that Kipper was Mr. Krupp's nephew, so Mr. Krupp arrived and sent the two to detention for "bullying Kipper."

11 days later, Mr. Krupp was unintentionally beaten up by his own nephew and his friends while escaping from the school due to a blackout.

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 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.

During The Ludicrous Lunacy of the Loopy Laserlightmare, Mr. Krupp cares about his job as Camp Director and delivers fake serveys to the F.U.N.C to keep it. In The Polarizing Plight of the Pitiless Poopetrators, he states that "he was the balloon to George and Harold's air" after losing his job. George then states that he made sense.

The name "Benny" was introduced in Captain Underpants and the Perilous Plot of Professor Poopypants, but it was revealed that it was a nickname in The Interactive Insanity of the Irritating Interlopers.