Greg is an image-conscious middle school student who begins keeping a journal to document his daily life, convinced that he will one day be rich and famous. He evaluates everyone based on their social ranking and constantly devises schemes to climb the popularity ladder. Preferring video games to physical exertion, Greg looks for shortcuts to success and consistently avoids taking personal accountability for his mistakes.

Rowley is Greg's sweet-tempered and fiercely loyal best friend. Unlike Greg, Rowley is completely comfortable in his own skin and maintains an innocent, juvenile outlook on life. He is an only child raised by intensely protective parents who tightly control his media consumption and activities. Because he genuinely values friendship, he endures Greg's manipulation, though his patience has limits.

Mr. Heffley is Greg's strict father. He values hard work, outdoor activities, and traditional masculinity, constantly pushing his sons to leave their video games and participate in sports. He wakes up at six o'clock every morning regardless of the day. He possesses a quick temper that flares when his sons disobey him, though his anger usually burns out as quickly as it ignites.

Mrs. Heffley is Greg's optimistic and nurturing mother. She focuses on her children's emotional growth, encouraging them to pursue diverse interests like school plays and music. When disciplining her children, she takes time to devise thoughtful punishments rather than reacting impulsively. Her earnest attempts to connect with her sons often clash with their rebellious preteen attitudes.

Rodrick is Greg's older brother and frequent tormentor. He plays the drums in a heavy metal band and actively avoids hard work by setting people's expectations for him as low as possible. He frequently pulls pranks on Greg, enjoying a lack of supervision and consequences that Greg heavily resents.

Manny is the youngest son of the Heffley family. He is heavily coddled by both his parents and enjoys complete immunity from punishment. He uses his status as the baby of the family to get his way, completely shielding himself from any consequences when he tells on his older brothers.

Fregley is a highly unusual student in Greg's class. Because he is the only student in Greg's weight class, he becomes Greg's default wrestling partner. His strange behavior, lack of personal hygiene, and hyperactive energy make Greg deeply uncomfortable.

Patty Farrell is a highly competent and observant student in Greg's class. She enforces the rules and ruins Greg's plan to cheat on a geography quiz by pointing out the classroom map. She also auditions for the lead role of Dorothy in the school play, making her a prime target for Greg's petty revenge.

Collin Lee is a student whom Greg views merely as a backup friend. When Rowley decides he needs a break from Greg's manipulation, Collin eagerly steps in to hang out with Rowley, sparking an intense, petty jealousy in Greg.

Mr. Jefferson is Rowley's highly protective father. He tightly monitors his son's environment, researching every video game on parent websites to ensure they contain no violence. He distrusts Greg and sees him as a bad influence, strictly limiting the boys' activities when they misbehave.

Mrs. Jefferson is Rowley's mother. She works closely with her husband to shield Rowley from any potential harm or negative influences. Her protective nature extends to altering her son's Halloween costumes to prioritize visibility and safety over the actual design.

Uncle Charlie is Greg's relative who consistently misunderstands what preteens want for Christmas. Instead of providing the video games Greg hopes for, Uncle Charlie presents Greg with a framed photograph of himself, completely missing the mark on holiday gifting.