Sheila is a ten-year-old girl living in New York City who relocates to Tarrytown for summer vacation. She secretly harbors intense, paralyzing fears of dogs, swimming, darkness, and spiders. To mask these deep insecurities, she adopts a bossy, know-it-all persona that often alienates her peers. She desperately wants to appear capable and fearless, though she feels deep shame over her phobias.

Libby is Sheila's thirteen-year-old sister who strives to appear sophisticated and grown-up. She often feels frustrated by her parents' constant accommodations for Sheila's anxieties. Like her younger sister, she battles deep personal insecurities, particularly regarding her physical appearance and her desire for validation from older teenagers.

Mouse is a confident ten-year-old local in Tarrytown who embraces her animal-themed nickname rather than her given name, Merle. As the junior yo-yo champion of the town, she is bold and totally unafraid to show her vulnerabilities. She extends genuine companionship to the new arrival and values honesty above saving face.

Mr. Tubman is a college professor who brings his family to stay at a colleague's house in Tarrytown for the summer. He shares an exceptionally close bond with his younger daughter and often goes out of his way to comfort her during nighttime panics. His tendency to indulge these fears causes significant tension with his older daughter.

Mrs. Tubman manages the household logistics for the summer relocation to the suburbs. While she accommodates many of her younger daughter's anxieties, she draws a hard boundary regarding water safety. She insists on mandatory swimming lessons and refuses to back down when faced with emotional outbursts.

Marty is a patient swimming instructor hired to teach a very reluctant student. He recognizes the genuine terror hiding behind combative behavior and adjusts his teaching methods accordingly. He relies on steady encouragement and clear communication rather than physical force.

Jennifer is a small dog belonging to the Egran family. She stays in the yard during the summer while her owners are away in England. Her simple presence acts as a constant stressor for the house's temporary residents, though she is entirely harmless and affectionate.

Jane is a local Tarrytown girl who quickly joins the neighborhood playgroup. She participates heavily in day camp activities and sleepovers, occasionally getting swept up in group conflicts and retaliatory pranks.

Sondra is another neighborhood peer who spends her summer with the local girls. She occasionally suffers minor injuries during their rough-and-tumble games but remains a loyal member of the day camp cohort.

Peter is a classmate from Manhattan who shares a long-standing mutual antagonism with his neighbor. His simple presence in confined spaces forces others to quickly invent excuses to avoid him.

Turtle is a large pet dog belonging to a young boy in New York City. His mere presence on an elevator is enough to send terrified neighbors fleeing for the stairs.

Maryann is a day camp attendee who suffers from severe stage fright during theatrical performances. She attends summer sleepovers and occasionally aligns herself with the older teenagers when conflicts erupt among the younger children.

Betsy is a four-year-old girl who copes with her canine allergies by inventing an imaginary dog named Ootch. Despite her young age, she possesses an impressive ability to dive off the board at the local pool.

Bobby is the twelve-year-old son of the Tarrytown homeowners. Though he spends his summer away in England, his highly territorial nature is felt through the aggressive warning notes he leaves behind regarding his extensive model airplane collection.

Hank is a fourteen-year-old aspiring filmmaker in Tarrytown. He becomes a fixture in the older social circles at camp and attends the neighborhood barbecue events at the end of the season.

Mumford is a stray-seeming male dog who frequently visits the yard to see his canine companion. His sudden appearances initially cause panic for the temporary residents before his true ownership and intentions are revealed.