George and Harold are the book’s protagonists. They are fourth-grade students and best friends. George loves to make up stories; he especially enjoys writing episodes about the superhero Captain Underpants. Harold loves to draw; he illustrates the comics that George writes. Both boys love jokes and pranks, which land them in trouble with Mr. Krupp. 

At the beginning of the book, George and Harold do not face the consequences of their actions. By the end, they show growth by rescuing Captain Underpants—the product of their greatest prank—from outlandish situations.

Even after their transformation, the boys continue to be fun-loving. Through them, Pilkey emphasizes that Mischief and Silliness Are Essential to Childhood.

Krupp is the principal at Jerome Horwitz Elementary School. He is a cruel leader who harshly disciplines the students he is supposed to be protecting. He is a foil to the fun-loving George and Harold, as he despises children, laughter, and joy.

Through Krupp, Pilkey explores the theme of Adults as Villainous Antagonists. He suggests that adults lose their sense of fun and magic as they age. Krupp stands in opposition to all that Pilkey holds out as valuable: silliness, mischief, and humor.