Moose is a twelve-year-old boy living on Alcatraz island, where his father works as a prison guard. He is highly protective of his older sister, Natalie, and works hard to manage the social expectations of his friend group. His secret, desperate plea to an infamous mobster for school admission help leaves him anxiously awaiting a demand for a return favor.

Natalie is Moose's sixteen-year-old sister who has autism. She attends the Esther P. Marinoff School in San Francisco, an arrangement made possible by a secret favor. She has a strong memory for objects, rules, and events, and possesses an extraordinary facility for numbers.

Cam is Moose and Natalie's father. He works as an electrician and prison guard on Alcatraz. He is a conscientious man who insists on telling the truth and doing the right thing, choosing to treat both his coworkers and the convicts with basic respect.

Capone is a famous, powerful mobster serving time in Alcatraz for tax evasion. He works in the prison laundry and maintains a mysterious degree of influence over the island's operations. He leaves coded messages in the laundry to call in his debts.

Darby is a loud, ill-tempered prison guard living in the civilian apartments. He resents the Flanagan family and actively looks for reasons to report them to the warden. He believes neurodivergent individuals should be institutionalized.

Piper is the pretty, self-centered daughter of the island's warden. She struggles with intense anxiety over her mother's high-risk pregnancy and feels neglected by her father. She frequently lashes out at Moose and his friends but relies on them for emotional support.

Jimmy is Moose's best friend on Alcatraz. He is unathletic and feels insecure about his poor baseball skills, preferring to spend his time studying science and training flies. He frequently feels overlooked when Moose spends time with other kids.

Annie is a girl Moose's age who lives on the island and excels at baseball. She holds Moose accountable for his dangerous choices and refuses to enable his secret-keeping, though she sometimes uses her baseball skills to make him jealous.

Theresa is Jimmy's seven-year-old sister. She is highly observant and keeps detailed files on the island's convicts. She feels intense guilt when her infant brother nearly chokes on a coin she gave him.

Warden Williams is the strict head of the Alcatraz penitentiary. He uses non-violent convicts as free household labor. He is intensely focused on the impending birth of his son, which leads him to neglect his daughter.

Buddy Boy is a convicted con artist who serves as a houseboy for Warden Williams. He uses his charm and skill at vocal mimicry to entertain the island children, though his smooth demeanor hides a calculating nature.

Willy One Arm is a former pickpocket serving time on Alcatraz. He works as a cook in the warden's house. He keeps a pet mouse named Molly in his pocket and performs tricks with his empty shirt sleeve to amuse the children.

Moose and Natalie's mother. She teaches piano and plays music at the officers' club. Her social life on the island flourishes when Natalie is away at school, though she remains highly invested in her daughter's progress and safety.

Bea is Darby's wife and Janet's mother. She runs the canteen where Jimmy works. She occasionally acts as a moderating force against her husband's anger, using her baking skills as leverage to calm him down.

Janet is the seven-year-old daughter of Darby and Bea Trixle. She plays on the island beaches and mimics her father's authoritative behavior by using a handmade megaphone.

Scout is Moose's school friend from the city. He is an excellent baseball player whose visits to Alcatraz create friction, as he inadvertently makes Jimmy feel inadequate and flirts with Piper.

Piper's mother and the warden's wife. She suffers from severe, life-threatening complications during her pregnancy, requiring hospitalization in the city and causing deep anxiety for her daughter.

A prison guard and father to Jimmy, Theresa, and Rocky. He is a steady presence in the civilian housing but faces sudden job insecurity when false allegations are made against him and Cam Flanagan.

Jimmy and Theresa's mother. She is a warm, community-minded woman who frequently bakes for the island residents. She encourages the children to forgive one another and act as a supportive family network.

A convicted axe murderer who performs plumbing repairs in the island's civilian apartments. His presence in the Flanagan home makes Moose highly anxious, as he suspects the convict is acting as a messenger.

Jimmy and Theresa's infant brother. A sudden medical emergency involving Rocky forces Moose to act quickly, leading to a reward that creates jealousy among his friends.

Al Capone's glamorous wife. Her visits to the island attract intense media scrutiny and require strict monitoring by the prison guards.

A small pet mouse kept by the convict Willy One Arm. She captures Natalie's intense focus during visits to the warden's house.