Sage is a brave sixteen-year-old girl with strawberry-blond hair and silver-blue eyes. She lives with her stepfather following her mother's death and her parents' divorce. Driven by a fierce determination to rescue her missing twin sister, she travels alone to a psychiatric facility on Staten Island, only to be mistaken for her twin and locked inside the horrifying conditions of Ward D.

Eddie is a young, handsome janitor who works at Willowbrook State School. Appearing more like a college student than an institutional worker, he is the only person on the staff who listens to Sage and believes she is not Rosemary. He expresses disgust at the abuse the residents suffer and quickly offers to help Sage prove her identity and contact her family on the outside.

Rosemary is Sage's identical twin sister who possesses schizophrenia and experiences delusions. Previously believed to be dead, she has actually been a patient at Willowbrook State School for six years. She is characterized as a kind, dreamy girl who always notices the small beauties of the world, though she suffers greatly under the institution's neglectful care before going missing from the grounds.

Alan is Sage and Rosemary's stepfather. He works at the local dump, drinks heavily, and maintains a cold, secretive demeanor. After Sage's mother dies, Alan actively lies to Sage for years, keeping Rosemary's confinement at Willowbrook a secret and subjecting Sage to his constant verbal and physical hostility.

Wayne is a bald, muscular attendant with tattooed arms who oversees the dayroom at Willowbrook. He uses a bully stick to threaten the women in his care and maintains a frightening, predatory presence. He actively exploits the vulnerable residents, using their cognitive disabilities and physical confinement to avoid any consequences for his predatory actions.

Dr. Baldwin is a Willowbrook psychiatrist responsible for overseeing patients in House Six. He wears thick glasses, has a fleshy pale face, and maintains a highly dismissive attitude toward the women under his authority. Rather than treating patients, he manipulates them, consistently ignoring Sage's desperate pleas about her true identity and authorizing her forced sedation and confinement.

Detective Sam Nolan is an investigator with the New York Police Department. He wears a wool coat, has unruly hair, and maintains stubble on his cheeks, showing more concern for his job than his physical appearance. Unlike the school's administrators, he is bothered by the rough treatment of patients and is willing to objectively evaluate Sage's claims.

Nurse Vic is the primary medical supervisor of the wards in House Six. She strictly enforces the institution's draconian rules and turns a blind eye to the abuses occurring right in front of her. When residents report misconduct, she dismissively redirects them or threatens them with severe punishments like isolated confinement.

Norma is an eighteen-year-old resident on Ward D and Rosemary's closest companion. She suffers from hallucinations and often reacts violently to her environment, slamming furniture and throwing objects. Her erratic behavior functions as a coping mechanism for the severe exploitation she endures at the hands of the ward's predatory staff.

Tina is a resident who has been confined to Willowbrook for eight years after suffering severe burn injuries. As one of Rosemary's previous acquaintances, she quickly accepts Sage under the belief that she is Rosemary. She actively tries to teach the newcomer how to survive the daily routines of the dayroom by eating quickly and avoiding the staff's attention.

Marla is a female attendant on Ward D who participates in the forceful subjugation of the residents. She orchestrates the degrading communal hygiene routines on the ward. She physically overpowers and chokes Sage when she tries to resist being stripped and processed with the other patients.

Evie is Dr. Baldwin's secretary at Willowbrook State School. She manages his administrative tasks, answers his calls, and actively prevents Sage from placing outgoing calls to the operator. She acts as a deliberate administrative barrier that keeps the patients completely isolated from the outside world.

Dr. Douglas Carter is a physician connected to the Willowbrook staff. He becomes emotionally volatile during a police investigation on the school grounds. He angrily threatens Dr. Baldwin and accuses the administration of causing direct harm to his family.

Dr. Wilkins is a medical professional at Willowbrook who breaks the institution's code of silence. Horrified by the squalid conditions and systemic neglect, he acts as a whistleblower. He grants a television camera crew secret access to House Six to document the horrors inside.

Hammond is the head administrator of Willowbrook State School. He prioritizes the institution's public image over the welfare of its vulnerable residents. He actively works to keep secrets and refuses to reinstate staff members who attempt to expose the facility's deplorable conditions.

Heather is one of Sage's friends from high school. She and Dawn frequently hang out near an abandoned tuberculosis hospital and share creepy urban legends about the local area. She inadvertently feeds Sage's fears of the dark and isolated corners of Staten Island.

Dawn is another of Sage's teenage friends from her normal life. She spends time with Sage at the local diner and shares rumors about the missing children in the area. She provides the backdrop of urban folklore that haunts Sage's thoughts during her journey.

Sergeant Clark is a New York Police Department officer who partners with Detective Nolan. He assists in questioning the Willowbrook staff. He physically examines the grounds when the police are called to investigate reports of strange occurrences at the facility.