Peter is a 12-year-old boy entering adolescence. He frequently feels irritated by his family's loud dynamic and acts highly protective of his dog, Turtle. Though he often projects frustration toward his younger brother, he demonstrates sudden maturity and resourcefulness when problems arise. He loves baseball, collects trading cards, and relies heavily on the support of his best friend during the shared vacation.

Fudge is Peter's highly active five-year-old brother. He possesses endless curiosity and a habit of taking vocabulary very literally. His energetic antics frequently lead to noisy situations, yet he manages to adapt to unfamiliar environments with immediate enthusiasm. He confidently treats his vacation as an opportunity to secure a future marriage, believing it will protect him from imaginary monsters.

Sheila is Peter's classmate who shares the Maine duplex with the Hatchers. She projects high confidence, frequently corrects others, and acts as a strict rule-follower. Underneath her prickly exterior, she shows surprising patience when interacting with younger children. She vocalizes her distaste for dirt and messes, though she occasionally softens her stance when left alone with animals she trusts.

Peter and Fudge's grandmother brings a youthful, vibrant energy to the Maine vacation. She acts as a stabilizing presence during stressful moments and offers a listening ear when Peter feels overwhelmed by his siblings. Her athletic nature and ability to stay calm during emergencies earn her widespread respect. She readily embraces new connections and encourages her family to appreciate every morning.

Jimmy is Peter's best friend who agrees to join the family vacation for a week. He shares Peter's sense of humor but usually remains more easygoing and adaptable to changes in plans. He occasionally feels embarrassed by his father's eccentric behavior and financial struggles, preferring to fit in with his peers.

Peter, Fudge, and Tootsie's parents try to maintain order while embracing the loud reality of their household. The mother tends to handle the daily schedules and discipline, while the father approaches the vacation with an excitable, adventurous spirit. They both encourage their children's creativity, even when it results in a ruined lawn or a messy afternoon.

Mitzi is a young girl vacationing in Maine who quickly bonds with Fudge. She shares his love for messy outdoor play and invents games while twirling in the mud. Her direct connection to a retired baseball player makes her an instant point of interest for Peter.

Jimmy's father is a stoic, struggling artist who focuses intensely on his creative work. He confidently claims the living room couch as his bed upon arriving in Maine, entirely surprising his hosts. He searches for creative inspiration outdoors and approaches family mishaps with unexpected artistic vision.

Sheila and Libby's grandfather joins the Tubman family for their summer vacation. He is an easygoing older man who finds humor in Fudge's unusual announcements and enjoys spending peaceful time outdoors away from the noise.

Tootsie is the one-year-old sister of Peter and Fudge. She is highly active, prone to throwing her oatmeal at unfortunate targets, and inadvertently leaves her physical mark on Mr. Fargo's artwork.

Sheila's older sister works at the local ice cream shop during the vacation. She often acts as a stern figure in the house, eagerly lecturing the younger children when their games become too loud for her liking.

Sheila and Libby's parents accompany the family on their summer trip to Maine. They share the cramped duplex with the Hatchers and participate in group outings alongside the rest of the noisy household.

Mrs. A is a warm, welcoming older woman who lives near the shore in Maine. She happily entertains Fudge's wild stories and provides a safe, comfortable stop for the boys during their foggy morning expedition.

A retired centerfielder for the Boston Red Sox. His active presence in Maine becomes a major source of excitement for Peter, who collects baseball cards and deeply idolizes the sport.

Isobel, affectionately nicknamed Izzy, is a sixteen-year-old library employee. She handles Fudge's dramatic temper tantrums with practiced kindness and quickly becomes the subject of Peter's intense romantic daydreams.

Peter's loyal pet dog accompanies the family on their summer trip. He provides a steady source of comfort for Peter but frequently repulses Sheila due to his strong, unfortunate odors.

Fudge's myna bird is highly capable of mimicking human speech. His loud vocabulary frequently annoys the older children, and his brief disappearance causes significant panic in the household.

Sheila's puppy is small, friendly, and notably the only dog she feels comfortable around. He quickly wins over Jimmy Fargo's affections upon his arrival.

The cheerful owner of the town's bicycle shop. He lightens a frustrating moment for Peter by officially welcoming him into the humorous "I Swallowed a Fly" club and recommending ice cream as a cure.