A shy, responsible almost-12-year-old girl who loves the safety and routine of her family's boardinghouse in Millerton. She prefers the company of adult boarders over her peers and feels socially awkward around other kids. Her quiet summer is disrupted when she learns of her hidden Uncle Adam's existence.

The 21-year-old youngest child of the Mercer family who has been institutionalized since age 12 due to mental health conditions. He is highly energetic and prone to wild mood swings. Often treated like a child by his parents, he forms an immediate bond with his niece, Hattie.

Hattie's prim, proper, and wealthy grandmother who cares deeply about social status and appearances in Millerton. She struggles to manage her son Adam and views him, as well as her daughter's boardinghouse business, as an embarrassment to the family name.

Hattie's mother, who rejected her wealthy upbringing to marry a middle-class artist and run a boardinghouse. She often acts as a mediator, caught between her own family's happiness and her mother's strict social expectations.

Hattie's artistic and industrious father who helps run the boardinghouse. He paints, makes animated home movies, and ignores his mother-in-law's snobbish disapproval of his career.

A wealthy, influential lawyer and Hattie's grandfather. Like his wife, he values decorum and public opinion, frequently losing his temper when Adam fails to follow social rules.

The niece of carnival owner Fred Carmel, she travels across the United States with her family. Used to a nomadic life surrounded by diverse performers, she is warm-hearted and easily accepts people for who they are.

An elderly, graceful woman who has lived at the boardinghouse since before Hattie was born. Surrounded by perfume bottles and sewing items, she serves as a comfortable, sympathetic grandmother figure to Hattie.

A retired clock repairman who lives at the boardinghouse. His room is filled with perfectly timed clocks, and though he is serious and busy, he approves of Hattie's responsible nature.

A chic and ambitious young bank secretary living at the boardinghouse. She dresses smartly and maintains a cosmopolitan air, representing the glamorous adult world that Hattie admires.

A classmate of Hattie's in Millerton who frequently bullies her. She represents the small-minded, judgmental attitudes of the town, quickly labeling anyone different as a freak.

One of Hattie's gossipy classmates who teams up with Nancy O'Neil to mock people they perceive as different.

Hattie's only school friend her own age, who spends every summer away in Maine.

The dedicated cook at the Owen family's boardinghouse who compliments Hattie on her diligence but wishes she would spend more time with kids her own age.

A recently widowed mother who moves her family into the boardinghouse's guest room in search of a fresh start and a job in Millerton.

Mrs. Strowsky's shy, red-haired daughter who is entering the seventh grade. She bonds with Hattie over their shared quiet natures.