Bett is a twelve-year-old girl from Venice, California, who loves surfing and animals. She is known for taking risks. Raised by her single father, Marlow, she is impulsive and highly independent. She initially resists the idea of her dad dating and expanding their family. She goes by the code name Dogfish.

Avery is a twelve-year-old from New York City who loves reading and writing. She prefers careful planning over spontaneity. Raised by her single father, Sam, she struggles with anxiety and a fear of the water. Avery values her comfortable routine and is highly skeptical of Bett's initial emails. She adopts the code name Night Owl.

Marlow is Bett's father and a builder who lives in a converted church in California. He is spontaneous and informal, enjoying outdoor activities and improvising his plans. Deeply devoted to his daughter, he hopes to blend his family with Sam Bloom's by sending the girls to the same summer camp.

Sam is Avery's father, an affluent architect living in New York City. He is highly organized, formal, and cautious. Sam wants Avery to befriend Bett but struggles to manage his daughter's resistance to change.

Betty, affectionately known as Gaga, is Bett's grandmother and Marlow's mother. Living in Texas, she is warm-hearted and generous. She possesses a naturally theatrical personality and cares deeply for her family.

Kristina is a theatrical and impulsive playwright who was best friends with Sam during college. She and Avery have been out of touch for years following a custody arrangement. She possesses an open personality that influences both girls when she reconnects with them.

Javier is a talented, young-at-heart ballet dancer originally from Cuba. Fluent in Spanish and English, he is a friend of Kristina's who becomes involved in the families' lives through the theater community in New York.

Dinos is a friendly, good-natured doorman who works at the building where Gaga and Kristina rent apartments. A true theater enthusiast, he quickly bonds with Gaga over coffee and her homemade candies.

Director Daniel oversees the camp where Bett and Avery are initially sent. He is an authority figure who adheres strictly to the camp's rules, struggling to manage Bett's rebellious behavior.

Brielle is an affluent camper who harbors prejudices regarding non-traditional family structures. Anxious about her parents' recent divorce, she projects her insecurities onto the other campers.

Tyler is Brielle's brother, attending a nearby boys' camp. Unlike his sister, he is open-minded and eager to make a good impression, demonstrating kindness and maturity in his interactions.

Mrs. Leonard is the director of a traditional summer camp in Maine. She is a strict enforcer of the camp's many long-standing traditions, including mandatory lake crossings and a ban on internet access.

Jilly is a camp counselor tasked with guiding the girls through rigorous outdoor activities. She is initially dismissive of Avery's severe anxieties about the water.

Sunny is Brielle and Tyler's mother. She is currently navigating a difficult divorce and holds conservative, sometimes judgmental views about other people's families.

Ben is Kristina Allenberry's intern, assisting the busy playwright with her correspondence and transportation during a theater festival.

Philip was Marlow's former partner. He passed away when Bett was very young, but he remains a foundational part of her family's origin story.

Judge Evan Balakian is the father of Avery's friend Ariel. He is a supportive community figure whom Avery contacts for legal and ceremonial advice.