Moose is a tall, responsible twelve-year-old boy who moves to Alcatraz Island with his family during the Great Depression. He is tasked with watching over his sister, Natalie, while trying to adjust to a new school and find normalcy through playing baseball. Though initially resentful of the move, his deep sense of duty and love for his family guide his actions.

Moose's sixteen-year-old sister who exhibits behaviors consistent with the autism spectrum. She finds comfort in counting, organizing her button box, reciting indexes, and eating lemon cake. She struggles with changes in routine and neurotypical social expectations, prompting her parents to seek specialized schooling for her in San Francisco.

Moose and Natalie's determined mother. She takes on extra work teaching piano to fund her mission of enrolling Natalie in the Esther P. Marinoff School. Desperate to avoid institutionalizing her daughter, she clings to extreme optimism, insisting the family pretend Natalie is ten years old to meet the school's strict age requirements.

Moose and Natalie's hardworking father who serves as an electrician and prison guard on Alcatraz. He works exhausting hours to support the family and pay for Natalie's potential specialized schooling. While he shares a quiet understanding with Moose, he often avoids domestic conflict by following his wife's rigid rules regarding Natalie's care.

The Warden's attractive, manipulative, and troublemaking daughter. Desperate to find a way off Alcatraz, she uses her position to orchestrate unethical schemes, such as selling convict laundry services to mainland classmates. She frequently breaks rules while maintaining a perfect facade in front of her father.

A talkative young girl who lives on Alcatraz. She appoints herself as Moose's unofficial guide, showing him around the island and creating informational cards about famous inmates. She shows an immediate, natural ability to interact calmly with Natalie without judgment.

Theresa's brother and one of the few boys Moose's age living on Alcatraz. Rather than playing sports, he spends his time building intricate contraptions out of household items, like a machine for launching marbles.

A quiet but observant girl living on Alcatraz. She loves playing baseball and frequently acts as a voice of reason, warning Moose about Piper's deceitful nature and the dangers of crossing the Warden's daughter.

The strict overseer of Alcatraz prison and Piper's father. He runs both the prison and the civilian aspects of the island with an iron fist, demanding precise compliance from everyone. He frequently uses his power over the guards' employment to keep their families in line.

A boy from Moose's mainland school in San Francisco who shares his passion for baseball. He represents Moose's desire for a normal social life off the island, though their friendship is frequently tested by Moose's childcare responsibilities and Piper's interference.

A prisoner assigned to gardening detail due to having only six months left on his sentence. He speaks with an overly familiar tone and forms a quiet, potentially concerning connection with Natalie.

A notorious real-life gangster incarcerated at Alcatraz. While physically unseen for most of the story, his presence looms large over the island's inhabitants and serves as the primary marketing tool for Piper's laundry scheme.

A special education tutor hired to help prepare Natalie for her second attempt at the Esther P. Marinoff School. She advocates for a strict approach, advising the Flanagans to remove Natalie's comfort objects to force her integration into neurotypical society.

The headmaster of the exclusive Esther P. Marinoff School. He evaluates whether Natalie is fit to stay in his program, prioritizing the comfort of the school's wealthy neighborhood over the immediate needs of struggling students.

Al Capone's elderly mother. Her gentle interaction with a baby on the ferry stands in stark contrast to the humiliating strip search she endures at the hands of the prison guards, profoundly shifting Moose's perspective on the inmates.

Moose's teacher at his new mainland school in San Francisco. She exercises casual cruelty by drawing negative attention to Moose's large size on his first day, establishing the social hierarchy of the classroom.

A student at the mainland school who falls for Piper's convict laundry scheme. He becomes furious when his returned clothes lack bullet holes or bloodstains, prompting his father to report the children to the Warden.

A prison guard at Alcatraz who suspects Piper's troublemaking but frequently lets her slide due to her father's position.