Jack is a five-year-old boy who has lived his entire life in a single 11-by-11-foot space he calls Room. He believes everything outside of Room is outer space and that the things he sees on television are entirely fake. He spends his days completing physical education exercises and playing with homemade toys like Labyrinth. He also frequently talks to personified objects like Wardrobe and Plant, forming deep attachments to his immediate surroundings. His understanding of reality relies entirely on what his mother teaches him.

Ma is a 26-year-old woman who has spent the last seven years locked in an enclosed space after being abducted at age nineteen. She dedicates all her energy to keeping her son safe and creating a structured, educational environment for him despite their extreme deprivation. She carefully controls what information Jack receives about the outside world to protect him from the reality of their situation. She suffers from chronic physical pain, including a rotting tooth and a badly healed wrist.

Old Nick is a man around fifty years old who abducted Ma by tricking her into helping him find his dog. He controls the keypad door to Room and brings essential groceries, occasionally cutting the power or threatening starvation when challenged. He visits at night to assault Ma while Jack is hidden away. He has recently been unemployed for six months, making him increasingly irritable about the cost of maintaining his prisoners.

Grandma is Ma's 59-year-old mother. She holds onto hope after her daughter disappears from a college campus. Her refusal to accept that her daughter is dead eventually causes a rift in her marriage, leading to a divorce. She is now remarried to a man named Leo and maintains traditional views on family and child-rearing.

Paul is Ma's brother, who grows up and moves forward with his life during the seven years she is missing. He works in a skyscraper office and is married with a young toddler. He is deeply emotional about his family and attempts to provide normal experiences for his relatives.

Grandpa is Ma's father, who believes his daughter died after she went missing. He divorces his wife over their conflicting beliefs about their daughter's fate and relocates to Australia. He struggles to process difficult emotional realities regarding what his daughter endured.

Leo is Grandma's second husband, whom Jack refers to as Steppa. He lives in Ma's childhood home with Grandma and assists in managing the household. He provides a patient and calming presence during tense moments with the family.

Dr. Clay is a medical professional at the Cumberland Clinic. He works to help patients process severe trauma and manage complex social adjustments. He also handles sensory processing issues and coordinates with other specialists to provide a safe, structured environment for recovery.

Noreen is a staff member at the clinic who provides daily care, meals, and assistance. She helps individuals adjust to overwhelming new environments by narrating their experiences and offering steady support to prevent sensory panic.

Ajeet is a man walking his dog, Raja. He becomes unexpectedly involved in a street-level conflict and demonstrates high civic responsibility. He persistently questions suspicious behavior and ultimately alerts the authorities when he senses a child is in danger.

Officer Oh is a police officer who responds to a civilian's emergency call. She uses gentle, patient questioning techniques to gather critical information from an uncommunicative child in distress.

Morris is a lawyer hired to protect his clients' privacy and navigate public scrutiny. He handles incoming mail, advises on potential lawsuits against aggressive media outlets, and firmly attempts to enforce boundaries with reporters and television producers.

Deana is Paul's wife and the mother of Bronwyn. She attempts to facilitate normal childhood activities but struggles to manage the unpredictable behaviors of multiple young children in public settings like shopping malls.

Bronwyn is the toddler daughter of Paul and Deana. She exhibits typical toddler behaviors, including throwing tantrums in stores when she cannot get the items she wants.

Walker is a young boy whom Jack encounters at a local library. He laughs when Jack crashes toy trains into his, providing Jack with his first real interaction with a peer outside of his family.

Raja is a small dog owned by Ajeet. Startled by a sudden commotion on the sidewalk, the dog reacts defensively out of fear during a chaotic encounter.