Greg is a middle-schooler and the middle child of the Heffley family. He is highly imaginative, frequently recording his thoughts and anxieties, but struggles with a lack of confidence and a tendency to avoid confrontation. Viewing himself as the victim of his circumstances, he is highly critical of his family members yet deeply reliant on them as they leave for a stressful summer camping trip.

Mom is the optimistic and hopeful driver of the Heffley family's emotional bond. She wears glasses and keeps her black hair done up, constantly striving to create picture-perfect memories. Her eagerness to document these moments for social media often clashes with the messy reality of her family, though she remains remarkably resilient when plans go awry.

Dad is a practical man who spends most of his time focused on work. During the family vacation, he handles traditionally male tasks like driving the camper and emptying the sewage tank. While he usually goes along with his wife's enthusiastic plans, he also demonstrates a desire for authentic, rugged camping experiences away from the crowds.

Manny is Greg's three-year-old younger brother. An exceptionally small toddler with large teeth, Manny dictates much of the family's schedule and activities due to his young age. Despite being a toddler, he possesses surprisingly advanced abilities and a knack for unexpected heroics when the family is in distress.

Rodrick is Greg's older brother, a tall and slim teenager with slightly thick hair and mischievous eyebrows. He is largely unimpressed with the family vacation and resents being forced into close proximity with his brothers. He frequently uses his status as the oldest sibling to harass and play cruel tricks on Greg.

Rowley is Greg's best friend from home. Though he does not attend the camping trip, he remains a fixture in Greg's mind as the intended audience for the exaggerated stories Greg plans to tell upon returning.

Big Marcus is one of the rowdy boys Greg meets at Campers' Eden. He and his friends spend more time wrestling and arguing than fishing, and they introduce Greg to various campsite secrets before pulling him into a messy prank.

Weevil is a camper at Campers' Eden who hangs out with Big Marcus. He becomes part of the group that bands together to take revenge on older teenagers using condiment-filled water guns.

Gramma is Greg's grandmother. She provides temporary housing for the Heffley family in her basement after a construction accident leaves them displaced and on a tight budget.

Great-Grandma is the catalyst for the family's summer vacation. She tasks Dad with disposing of an old camper that belonged to Greg's uncle, which the family instead repurposes for their trip.

The camp director is the main authority figure at Campers' Eden. When a prank involving condiments goes wrong, the local boys point the finger at Greg, causing Greg to panic and hide at the bottom of the pool when the director appears.

A fellow guest at Campers' Eden, this unnamed man makes a habit of disturbing the Heffleys. He creates loud noise early in the morning with a chainsaw, steals Greg's shower stall when Greg's quarter runs out, and dumps the family's laundry on the laundromat floor.