Beatrix is an 11-year-old English girl sent away from London to escape the dangers of World War II. She stays with the Gregory family in Boston and spends summers on their island in Maine. Stoic and observant, she adapts quickly to her new surroundings but carries quiet guilt about her fading memories of her parents. She possesses a quiet intelligence and focuses intensely on the relationships around her.

William is the 13-year-old older son of the Gregory family. He harbors an independent streak and strongly desires to contribute to the war effort by taking aircraft lookout shifts at his school. He frequently clashes with his parents regarding his future, specifically his reluctance to attend Harvard and follow the family legacy. He connects deeply with Beatrix over a shared sense of independence and maturity.

Gerald is the nine-year-old younger son of the Gregory family. Unlike his older brother, he is immediately open with his affection and enthusiastically welcomes Beatrix into their home. He tracks the progress of the war carefully through newspapers. He demonstrates a natural empathy for others and possesses a laid-back, easygoing personality that balances the tension in his household.

Millie is Beatrix's mother, a working-class woman in London who holds down bookkeeping work and drives ambulances during the war. She struggles deeply with the physical separation from her daughter and worries constantly that Beatrix will eventually forget her. Her intense anxiety about the evacuation creates significant strain in her marriage and fuels a quiet resentment toward the American foster family.

Nancy is the wealthy, warm-hearted matriarch of the Gregory family. Having grown up with staff and a fancy lifestyle, she now focuses her energy on maintaining her household in Boston and cooking for her family. She instantly falls in love with Beatrix, eagerly taking on maternal duties like drawing her evening baths and buying her clothes. Her effervescent personality often contrasts with her husband's quiet demeanor.

Reginald is Beatrix's father, a pragmatic man who initiates the plan to send his daughter to the United States to keep her safe from bombings. He joins the Home Guard to contribute to the war effort and avoid the growing emotional tension in his household. He believes entirely in his decision to evacuate Beatrix, even as it damages his relationship with his wife.

Ethan is the stoic patriarch of the Gregory family and a math teacher at a boys' school. He is a quiet, reserved man who prefers intellectual connection over emotional displays. He takes responsibility for teaching Beatrix how to swim during their summers in Maine and connects with her father through transatlantic letters, discussing politics and playing postal chess.

Rose is a young woman from Boston who eventually enters the Gregory family's orbit as William's romantic partner. She holds onto high expectations for her life and dreams of a comfortable, elegant future similar to the wealthy families she admires. Her practical desires and focus on stability often conflict with the nostalgic, restless nature of the Gregory men.

Julia is a woman in London who partners with Millie to drive ambulances during the wartime bombings. She serves as a sounding board for Millie, listening to her worries about Beatrix. She provides practical support and companionship during difficult, high-stress shifts in the city.

Gertrude is Millie's mother, living in London. She observes the toll the overseas separation takes on her daughter and attempts to offer practical financial solutions to bridge the distance between Millie and Beatrix.

Tommy is a handsome young fighter pilot stationed in London during the war. He possesses useful military connections and a confident demeanor, stepping into Millie's life during a period of intense loneliness, fear, and transition.