Greg is a middle school student who prioritizes popularity above almost everything else. He often looks for shortcuts in his schoolwork and relies on his best friend, Rowley, for social leverage. Despite his desire to be admired, his insecurity and selfishness frequently lead him to make poor choices, including bullying others and lying to his parents.

Rodrick is Greg's older brother and the drummer for the heavy metal band Löded Diper. He actively avoids responsibilities like schoolwork and chores, preferring to manipulate his parents into doing tasks for him. He holds a secret about Greg's embarrassing summer incident, using it to continually blackmail his younger brother.

Rowley is Greg's best friend and an only child who retains a strong sense of childlike innocence. He has no interest in climbing the middle school social ladder, yet he naturally attracts positive attention by simply being himself. He enjoys playing with action figures and deeply admires Greg, asking for his own diary to match his friend.

Mrs. Heffley is Greg's mother and an extroverted parent who desperately wants her children to get along. She introduces various schemes, such as the Mom Bucks currency and mandatory drum lessons, to encourage responsibility and harmony. She defends her children's interests fiercely, even when her husband disapproves.

Mr. Heffley is Greg's introverted father who prefers peace and quiet. He retreats to work on his miniature Civil War battlefield whenever the house becomes too chaotic. He holds traditional standards for masculinity, making him highly critical of Rodrick's heavy metal band and lack of academic ambition.

Manny is the youngest sibling in the Heffley family. He is heavily sheltered by his parents, completely escaping the harsh discipline Greg receives. He frequently ruins his brothers' belongings and tattles on them without facing any consequences.

Chirag is a middle school student who becomes the target of a cruel school-wide prank orchestrated by Greg. When he returns to school, the entire class pretends they cannot see or hear him. Despite the extensive bullying, he is surprisingly quick to forgive Greg when adults intervene.

The boys' grandfather lives in a senior living facility called Leisure Towers. He serves watercress salad, plays old board games, and keeps his television constantly tuned to the building's lobby security camera.

Bill is a member of Rodrick's heavy metal band. He is a man in his mid-30s who still lives with his parents, a fact that deeply alarms the Heffley family regarding Rodrick's potential future.

Leland is a high school student who acts as Rowley's new babysitter. He teaches his young charges how to play a game called Magick and Monsters, inadvertently drawing the intense interest of Mrs. Heffley.

Scotty is a first grader who takes karate classes. He plans to perform in the upcoming talent show, eventually requiring an older student to step in as his sparkly-costumed assistant due to an injury.

Gramma is the matriarch of the Heffley family. She expects hard work when she employs her grandsons to clean up her yard, but she is left unhappy when they attempt to cheat the system and abandon the job early.