Madeline is an 18-year-old living in a California suburb who has spent her life confined to her house due to a rare autoimmune disorder. She relies entirely on books and online schooling to experience the world. She maintains a highly structured, monotonous daily routine with her mother and her nurse. When a new family moves in next door, she becomes deeply curious about the teenage boy, prompting her to seek connection beyond her isolated environment.

Olly is the teenage boy who moves into the house next door to Madeline. He frequently wears black clothing and uses parkour to move around his environment, spending time climbing on his roof. He possesses a vibrant physical energy that contrasts with Madeline's intellectual, confined life. He often seeks refuge outside his home to avoid his father's volatile behavior.

Pauline is a physician and Madeline's primary medical caregiver. She is highly conscientious, strictly enforcing a sterile environment and a punctual daily schedule to protect her daughter from outside contaminants. Following the deaths of her husband and son in a car crash, she centers her entire existence around keeping Madeline safe.

Mr. Bright is Olly and Kara's father. Following a job loss in New York City, he developed a severe alcohol dependency. He is verbally hostile and prone to angry outbursts, creating an atmosphere of fear and chaos within his household.

Carla is the nurse who has cared for Madeline for most of her life. She spends her days monitoring Madeline's vitals and providing companionship. She encourages Madeline to experience life rather than just read about it, often acting as a mediator who helps the teenager explore boundaries.

Mrs. Bright is Olly and Kara's mother. She primarily stays at home and attempts to manage the volatile environment created by her husband's drinking. She acts submissively, often covering up her children's rule-breaking to prevent further conflict in the household.

Kara is Olly's teenage sister. She reacts to her family's dysfunction by withdrawing from them. She spends her time smoking cigarettes in the yard and heavily using her cell phone to distance herself from her home environment.

Rosa is Carla's 17-year-old daughter. She is currently experiencing a rebellious phase, which causes tension and distance between her and her mother.

Zach is a friend of Olly's from New York. He is an aspiring rock performer who has not yet come out to his parents about his sexuality. He provides a connection to Olly's past before the Bright family relocated to California.

Janet is a nurse hired to monitor Madeline's health. Unlike Carla, she maintains a strict, unfriendly demeanor and immediately establishes a hostile, purely professional dynamic with her patient.