Greg is a sixth-grader who writes and illustrates a diary about his middle school experiences. He views his sense of humor as his greatest asset and relies on it to build his social status. He actively avoids physical exertion and chores, leading to a prolonged crisis when he refuses to wash his own laundry. He approaches his friendships transactionally and constantly seeks shortcuts to avoid difficult tasks.

Rowley is a cheerful sixth-grader who happily engages in playful activities with younger children. He lacks Greg's obsession with maturity and social status, possessing a trusting demeanor that makes him vulnerable to manipulation. He remains emotionally open and easily hurt, crying when he gets a splinter during Boy Scouts.

Frank expects his sons to embody traditional masculine ideals of physical strength and discipline. He frequently clashes with Greg over his laziness and poor athletic performance. His desire to send Greg to military school stems from his anxiety about how his family appears to others, particularly his employer. He possesses his own vulnerabilities, actively dreading public performances.

Rodrick is Greg's rebellious teenage older brother who plays in a heavy metal band called Löded Diaper. He uses his age and size to intimidate Greg, subjecting him to pranks and physical bullying. He ignores his parents' attempts to enforce rules and maintains a messy lifestyle that Greg fears adopting.

Susan acts as the mediator in the Heffley household, stepping in to stop conflicts between her three sons. She enforces new responsibilities, such as requiring Greg to do his own laundry, while remaining highly attuned to social conventions. She occasionally embarrasses Greg in public by treating him like a young child or wearing inappropriate gym clothes to his school.

Manny is Greg's toddler brother who communicates primarily in nonsense words like the insult Ploopy. He receives special treatment from his parents and avoids punishment for behaviors that would get his older brothers in trouble. He struggles to give up baby habits like his pacifier and his worn blanket, Tingy.

Holly is a popular sixth-grader who participates in social activities like roller skating and sleepovers. She exhibits a cheerful nature but pays little attention to Greg's comedic routines. She remains largely unaware of Greg's intense focus on her.

Lenwood is a teenager from the Heffleys' neighborhood who historically caused problems for Frank. He recently attended Spag Union Military Academy, which drastically improved his behavior and turned him into an upstanding citizen working at the local movie theater. His transformation directly inspires Frank's threats toward Greg.

Fregley is an off-putting classmate whom Greg considers socially beneath him. He behaves unpredictably at school events, ambushing Greg at the Valentine's Dance and ruining an opportunity to speak with Holly.

Mr. Warren is Frank's employer. He has highly athletic sons, creating a standard of physical fitness that Frank desperately wants his own children to meet.

Mr. Litch is the demanding coach of Greg's recreational soccer team, the Red Socks. He yells at his players and punishes Greg for attempting to crack jokes during wind sprints.

Alex is a highly academic student whom Greg hires for five dollars to investigate a stolen teacher's dictionary. He quickly solves the mystery but keeps the payment for himself.

Patty is a serious student who takes her temporary authority as class monitor very strictly, ensuring nobody leaves the room during recess.

Uncle Charlie is a relative who inadvertently causes Greg's chore crisis by gifting him a laundry-tossing game. He previously gifted Greg an embarrassing pair of Wonder Woman Underoos.

Leon is a school bully who flicks Greg's ears during detention. He receives additional punishment when Greg tricks the teacher into thinking Leon struck him.

Chirag is a student whom Greg attempts to prank by pretending the entire class cannot hear him speak. He quickly figures out the trick and reports the behavior.

Tucker is the soccer team's primary goalie. His sudden hand injury forces Greg to step off the bench and defend the net during a crucial game.

Mr. Snella films party guests performing silly antics in front of his baby, hoping to submit the footage to a television show.

Mrs. Craig is a teacher who holds her entire class inside during recess for several days until someone confesses to stealing her dictionary.

Corey is a student who accidentally takes the fall for the dictionary theft when the teacher spots him holding the book.

Ward is Rodrick's teenage friend who hangs out at the Heffley house.