Fern Castle is a 27-year-old librarian living in Melbourne. She has sensory processing issues that make loud noises, bright lights, and unpredictable social situations physically uncomfortable for her. She manages her environment using swimming goggles and earplugs to block out excess stimuli. She relies heavily on her twin sister to help her interpret social cues and organize her life. Her job at the library provides her with a structured, comforting environment where she excels.

Rose Castle is a 27-year-old interior decorator and Fern's fraternal twin. She manages type 1 diabetes and considers herself a perfectionist with a highly organized home. Following a difficult childhood and her mother's institutionalization, Rose took on a highly protective parental role toward Fern. She is currently dealing with marital difficulties and the painful realization that she may not be able to conceive a child.

Nina Castle is the mother of the twins. She suffered severe brain damage from an overdose of Valium and alcohol when the girls were twelve years old. She currently lives in a permanent care facility where she receives various therapies. Her past actions impact her daughters heavily, with their memories of her parenting differing significantly.

Rocco Ryan/Wally is an American tech entrepreneur who recently moved to Australia for a fresh start. He chooses to live out of a van and maintains a highly structured daily routine. He shares certain neurodivergent traits with Fern, making communication between them straightforward and honest. He previously developed a highly successful restaurant delivery app before seeking a quieter lifestyle.

Owen is Rose's husband. He has recently separated from Rose, moving away during a period of marital strain. Rose attributes his departure to their ongoing struggles with fertility and her overwhelming desire to have a baby.

Carmel is the new manager at the Bayside Public Library. She is eager to build a cohesive team and initially struggles to understand Fern's reluctance to participate in social events like bowling. She values her staff and tries to encourage them to learn new skills.

Gayle is a librarian working alongside Fern. She understands Fern's literal communication style and frequently steps in to assist when interactions with library patrons become socially complicated.

Linda is another employee at the Bayside Public Library. She participates in workplace social events and shows friendly interest in her coworkers' lives.

Janet is Fern's former boss at the library. She recognized Fern's need for a quiet space during busy periods and showed her a small secret room in the back to use during sensory overloads.

Mrs. Delahunty was the local librarian when Fern and Rose were five years old. She took a strong interest in the girls, teaching them advanced vocabulary and quizzing them on books while their mother brought them to the library daily.

Daniel is a former commercial airline pilot and an ex-boyfriend of Nina Castle. He seemed like a stable, fatherly figure to the twins and organized a memorable camping trip for the family.

Billy was Daniel's fourteen-year-old son. He joined the Castle family on a camping trip when the twins were twelve. He was competitive and playful, but his presence ended in a tragic drowning incident that deeply affected both sisters.

Gary is a former boyfriend of Nina Castle from when the twins were eleven. He was an abusive presence in their home, and his actions left a lasting, traumatic impact on Rose.

Mrs. Hazelbury is Fern's neighbor. She is nosy and quick to quote municipal bylaws regarding parking and pets to anyone who will listen.

Teresa is a speech therapist working at the Sun Meadows care facility. She uses cranial magnetic stimulation to help Nina regain some of her lost vocabulary.

Alfie is Rose's pet cavoodle. He is well-behaved but ends up in distress when left unattended due to a miscommunication between the sisters.