Mr. Krupp is a fictional character from the popular children's book series, Captain Underpants, created by author Dav Pilkey. He is the principal of Jerome Horwitz Elementary School and is known for his strict and often grumpy demeanor. His character serves as an antagonist to the main characters, George Beard and Harold Hutchins. In the series, Mr. Krupp undergoes a transformation into Captain Underpants, a superhero who fights against evil while wearing only a cape and underwear. This transformation occurs when George and Harold use a magic ring to hypnotize him. Mr. Krupp's character adds humor and excitement to the stories, making him a memorable figure in children's literature.