Greg is a middle-school student who records his daily life and imaginative schemes in a personal diary. He constantly seeks popularity and easy solutions to his problems, avoiding hard work or household chores whenever possible. His vivid imagination frequently causes trouble, leading to elaborate plans that rarely work out as intended.

Rowley is a kind-hearted and somewhat passive middle-school student. He defers to Greg's opinions and often participates in his friend's schemes, even when they make him uncomfortable. He prefers reading books meant for younger children and easily gets frightened by horror stories.

Greg's mother is an optimistic woman who decides to return to college to complete her master's degree. She values academic achievement and constantly tries to nurture her sons' potential, though she is frequently disappointed by their lack of motivation. She sets up household chore systems and encourages her children to broaden their horizons.

Greg's father takes a somewhat passive role in parenting but strongly values perseverance. He frequently doubts his sons' commitments, preferring practical solutions over expensive investments in their fleeting hobbies. He uses grounding as a primary discipline method when his sons inevitably misbehave.

Rodrick is a high school student who prefers napping and playing in his band over planning for college or doing chores. He frequently torments his younger brother Greg by telling him elaborate lies and stealing his birthday money. Despite his lazy demeanor, he occasionally steps up to help his family in a crisis.

Maddox is a boy from a neighboring town who grows up without access to television or video games. He spends his free time playing the violin, reading, and building massive, intricate Lego structures. His structured, screen-free upbringing contrasts sharply with Greg's lifestyle.

Manny is the youngest sibling in the Heffley family. He plays on a terrible soccer team but still receives participation trophies, which annoys Greg. He owns a learning toy that the family pig uses to communicate.

Mrs. Selsam is Maddox's mother, who lives in a neighboring town. She strictly prohibits her son from eating junk food or using screens, actively interrupting when she catches Greg showing Maddox a computer game.

Mariana is a popular girl in Greg's grade whose parties are legendary among the student body. Because her previous party got out of hand, she restricts her guest list to students who are in the school band with her.

The Heffley family's pet pig is surprisingly intelligent, occasionally putting together full sentences on Manny's toy. The pig possesses a voracious appetite and causes chaos by consuming an entire jar of candy corn.

Mrs. Mott is Greg's reading teacher. She forces her students to stay inside during recess after almost the entire class submits inadequate biographies on a fictional horror author.

Evan is a student who plays the French horn alongside Greg. Greg plans to rely on him during the Fall Concert, but Evan misses the performance to get his braces removed.