C.J. is a thirteen-year-old eighth grader known around Canaan as a daredevil. He frequently executes risky stunts on his bicycle and skateboard to intentionally cause accidents. These public falls serve as a necessary cover story for the bruises and injuries inflicted by his violently abusive stepfather. Unwilling to share his difficult home life with his friends, he uses the newly discovered underground bunker as a safe haven.

Ricky is the new kid in Canaan and is a year younger than his eighth-grade classmates because his high intelligence allowed him to skip a grade. Having previously attended a magnet school, he faces resentment from his peers. He desperately wants to fit in with Evan's established friend group and earns a place among them when he discovers the metal hatch leading to the underground shelter.

Evan is an eighth grader living with his grandmother after his drug-addicted parents abandoned him and his older brother, Luke. Feeling cast aside by his parents and increasingly alienated from his brother, Evan relies heavily on his close-knit group of friends. He initially resents having to entertain Ricky before the group bonds over the bunker.

Mitchell is an eighth grader who struggles with obsessive-compulsive disorder and academic difficulties. He is highly anxious and adheres strictly to personal rules, such as avoiding the number thirteen. His family struggles financially after his mother loses her factory job and health insurance, causing Mitchell to abruptly lose access to his therapy sessions.

Jason is an eighth grader caught in the middle of his parents' bitter divorce and joint custody battle. He divides his time evenly between his parents' separate residences, feeling treated like a possession rather than a son. To cope with his unstable home life, he centers his world entirely around his relationship with his girlfriend, Janelle.

Jaeger is a sixteen-year-old high school dropout who drives around Canaan in a noisy red Mustang. He bullies younger children and exploits anyone he perceives as weaker. He takes a keen interest in Evan's older brother, Luke, using the younger boy as a sidekick while trying to uncover where Evan and his friends are getting extra money.

Marcus is Evelyn Sciutto's second husband and C.J.'s stepfather. Standing six-foot-three, the former semiprofessional wrestler projects the image of a perfect, generous father to the outside world. In reality, he possesses a violent temper and physically abuses both his wife and stepson, frequently purchasing expensive gifts afterward to cover up his behavior.

Janelle is a seventh-grade student and Jason's girlfriend. As the daughter of a local police officer, she is observant, tough, and refuses to be intimidated by others. She demands honesty in her relationship with Jason, which creates friction when he attempts to keep the existence of the newly discovered bunker a secret from her.

Luke is Evan's sixteen-year-old brother who lives with their grandmother after their drug-addicted parents abandoned them. Believing that toughness is the only way to survive his circumstances, Luke distances himself from his family and allies himself with the older, delinquent Jaeger Devlin.

Evelyn is C.J.'s mother and Marcus's wife. She copes with her husband's violent outbursts through severe denial, ignoring the obvious causes of her son's injuries and her own bruises. She desperately tries to maintain the illusion of a happy, functioning family and convinces herself that Marcus is improving.

Dr. Breckinridge is a psychiatrist who previously treated Mitchell for his obsessive-compulsive disorder. He takes immense pride in his backyard herb garden. Mitchell believes the doctor dropped him as a patient, though the termination was actually due to Mitchell's mother losing her health insurance.

Grandma Donnelly is the guardian of Evan and Luke. She took the boys in after their parents abandoned them due to drug addiction. She is often critical of Luke's behavior and his association with Jaeger, while showing more silent sympathy for the younger Evan.

Bennett Delamere is the deceased, wealthy eccentric who originally constructed the underground fallout shelter in the 1970s. Anticipating World War III, he stocked the thirty-foot living space with canned goods, a chemical toilet, vintage videotapes, and silver utensils.