Amy is a third-year medical student beginning an overnight shift on her hospital's locked psychiatric floor. She carries lingering trauma and fear regarding the ward, stemming from a visit eight years prior when her childhood best friend was admitted there. Diligent and eager to please, she struggles with her own anxieties and hidden insecurities while working alongside her ex-boyfriend.

Jade is a patient on Ward D diagnosed with Bipolar I. Once a studious and bright teenager, she began exhibiting erratic and impulsive behavior at sixteen, which led to her initial hospitalization. She harbors deep resentment toward Amy for a perceived abandonment eight years ago, viewing her former friend's medical success with bitter irony.

Will is a young patient on the ward diagnosed with paranoid schizophrenia. He was admitted to the emergency room after claiming to hear voices. Rather than displaying typical distress, he appears surprisingly lucid and polite. He prefers reading over discussing his symptoms, leaving his assigned medical student uncertain of how to approach him.

Cameron is a fellow medical student and Amy's recent ex-boyfriend. He broke off their relationship to focus entirely on studying for his board exams. Highly ambitious and eager to impress authority figures, he actively flatters the attending physician while simultaneously using the overnight shift as an opportunity to reconcile with a frustrated Amy.

Dr. Beck is the young, attractive physician on call for the overnight shift in Ward D. He provides the medical students with an orientation of the locked floor, explaining the security protocols and assigning them patients to interview. He acts as a mentor figure for the shift, presenting a patient demeanor.

Ramona is the nurse on duty during the overnight shift. She assists Dr. Beck in maintaining order on the locked floor, handling patient charts, and managing the physical safety of the residents.

Damon is an unseen, highly violent patient securely confined to Seclusion One. Staff and other patients frequently discuss his dangerous reputation and history of violence, building an atmosphere of fear around his locked room. He is scheduled to be transferred to a more secure facility in the morning.

Daniel Ludwig, affectionately nicknamed Spider-Dan by the hospital staff, is a patient diagnosed with schizophrenia. He firmly believes that he is the comic book superhero Spider-Man. He is largely cooperative but frequently discusses his web-shooters, becoming Cameron's chosen interview subject for the night.

Miguel is an anxious patient diagnosed with schizophrenia who believes God is his father. He spends much of his time frantically trying to find a way to leave the hospital, frequently hovering near the main doors and tampering with the exit keypad.

Mary is an elderly patient on the psychiatric floor who spends her time constantly knitting. She is generally quiet but occasionally experiences intense periods of distress. She carries metal knitting needles that she claims are for her own protection.

Gabby is Amy's roommate and a fellow medical student who has already completed her own psychiatry rotation in Ward D. She provides transportation to the hospital and attempts to reassure her nervous friend about the safety of the locked floor.

Mrs. Carpenter is Jade's mother, seen primarily in Amy's memories from eight years ago. She works long hours as a waitress and suffers from bipolar disorder, creating a volatile and stressful home environment for her teenage daughter.

The Young Girl is a recurring figure who only Amy can see, beginning when Amy was sixteen. Appearing as a child in a dress, she acts as a physical manifestation of Amy's darkest impulses, urging her to shoplift, cheat, or make destructive choices.