Moose's 16-year-old sister, whose extraordinary abilities with numbers and memory help to highlight her intellectual powers. Her neurodivergence poses unique challenges for her and her family on Alcatraz Island.

Moose's compassionate and brave father, who works as a prison guard and electrician at Alcatraz. His teachings emphasize truth and doing the right thing, influencing Moose's moral outlook.

The notorious mobster incarcerated on Alcatraz, who wields influence both inside and outside the prison despite his confinement. Known for his charisma, Capone engages with Moose in subtle ways that affect Moose's life significantly.

An Alcatraz prison guard and the story's main antagonist, known for his harsh, callous demeanor and his animosity toward the Flanagan family. He is intent on enforcing a punitive approach to prisoners and situations on the island.

Darby's daughter and Moose's girlfriend, who struggles with a challenging home life and often acts out. Throughout the story, she confronts her feelings and relationships, impacting those around her.