Thirteen-year-old Moose lives on Alcatraz Island and carries the heavy responsibility of watching out for his older sister, Natalie. He worries constantly about his father's new promotion to associate warden, which puts a target on his back among the inmates. Moose is tall and possesses a strong pitching arm for baseball. He teams up with other children on the island to solve problems and protect his family.

Moose's sixteen-year-old sister has an undiagnosed developmental disability that makes socializing difficult and causes her to avoid eye contact. She approaches the world in her own distinct way and shows profound talent for numbers and counting. She relies heavily on her family's care, though she desires more independence than her mother typically allows.

Cam is Moose's father, recently promoted from electrician to associate warden at Alcatraz. He is a gentle, compassionate man who believes in rehabilitating prisoners rather than merely punishing them. His slender build and polite demeanor cause his family to worry about his physical safety among the island's violent convicts.

Known as "Double Tough," Darby is a belligerent prison guard who believes strongly in harsh punishment over rehabilitation. He deeply resents Cam Flanagan for receiving the associate warden promotion he wanted for himself. He takes every opportunity to challenge Cam's authority and treats the Flanagan family with open disdain.

Piper is the warden's pretty thirteen-year-old daughter. She flaunts her insider knowledge of the prison and demands obedience from the other children. Living in an imposing mansion isolated from the civilian kids, she relies on favors and gifts to secure social influence.

Donny is a stylish, slender man in his twenties who regularly visits Alcatraz to have his laundry done for free. As the son of a former guard's widow who remains on the island, he treats the residents like close friends. He frequently chats with the island kids, projecting a charismatic and easygoing presence.

Annie is Moose's blonde-haired best friend and frequent baseball companion. She possesses a steady, empathetic nature and excellent logic skills. Her grounded personality provides a reliable contrast to Piper's unpredictability, making her a crucial source of support for Moose.

Moose and Natalie's mother teaches piano in the city and works hard to manage the family's stressful home life. She worries intensely about Natalie's behavior in public and tries to shield her daughter from the judgment of critical neighbors.

Jimmy is a resident of the 64 building and a reliable friend to Moose. He has a keen interest in quirky science experiments, like growing mold in his navel. He works at Bea Trixle's grocery store and frequently assists Moose in investigating strange events on the island.

Bea is Darby's wife and runs the island's grocery store. She is quick to pass judgment and frequently uses cruel language when speaking about Natalie's developmental condition. She actively spreads rumors and looks for scapegoats whenever accidents occur.

The mother of Jimmy and Theresa, Mrs. Mattaman is a warm and sympathetic neighbor. She provides comfort to the Flanagan children during crises and offers Moose sensible advice about managing his heavy emotional burdens.

A notorious mobster serving time at Alcatraz, Capone exerts a strange pull over the island's culture. Though locked away, he manages to communicate subtly with the outside world, occasionally intervening in Moose's life through mysterious notes.

Theresa is Jimmy's eight-year-old sister who runs errands and performs chores for Piper in exchange for gifts. She is inquisitive and eager to be included in the older children's activities.

Janet is the seven-year-old daughter of Darby and Bea. She spends much of her time playing with imaginary pixie friends, creating makeshift homes for them out of hatboxes.

The head warden of Alcatraz projects an intimidating, authoritative presence. He prefers to manage his responsibilities from his imposing mansion near the cellblock, leaving much of the daily operations to his subordinates.

Annie's father is an active member of the Alcatraz civilian community. He steps up during emergencies to help his neighbors but remains wary of the tensions surrounding the prison administration.

Mr. Mattaman is the father of Jimmy and Theresa. He partakes in island social events, including neighborhood poker games, though his family sometimes worries about their financial situation.

Mrs. Kelly is a no-nonsense tutor who travels to the island to teach Natalie social skills. She firmly believes in preparing the Flanagan children for the long-term realities of managing a developmental disability.

Mrs. Caconi is a widow who lives on Alcatraz through the charity of the warden. She relies on her adult son Donny, who visits frequently to handle their laundry and check in on her.

Count Lustig is a world-famous confidence man incarcerated at Alcatraz. He maintains a surprisingly ordinary appearance compared to the other prisoners but possesses a long history of elaborate frauds.

Indiana is a menacing, chinless convict who shaves his eyebrows. He openly displays his hostility toward the prison staff and specifically targets the new associate warden with disrespectful gestures.

Lizard is an imposing convict who earned his nickname from a rumor about his unusual eating habits. He frequently runs interference for his fellow inmates during work details.