Sixth-grader Greg is the protagonist and narrator of the novel. He is also the presumed illustrator of the cartoons in the novel and portrays himself as fairly small and skinny, with three hairs on his head, indicating both his irreverent humor and his underlying insecurity about his appearance. Despite this, Greg has a high opinion of himself and views his sense of humor as his best personal asset and his ticket to popularity.

This self-assuredness is central to Greg’s character, though it often leads him to engage in morally gray behavior. He views relationships through a transactional lens, constantly evaluating what he can gain from those around him. For example, Greg goes out of his way to be nice to his family members around the holidays, but this is only to ensure that he receives good Christmas presents. His actions are driven more by self-interest than by genuine care for others, highlighting his willingness to manipulate situations for his own benefit. He also lies regularly to his friends and family as well as to the reader, positioning him as an unreliable narrator at times. Greg is often considered to be an anti-hero because of these morally gray behaviors, though Kinney considers Greg to be more of an ordinary flawed kid.