Matthew "Moose" Flanagan - 12 year old boy who plays baseball and cares for his sister and would do anything to help her.
Natalie Flanagan - Moose's 16 year old autistic sister; who causes controversy throughout the book. Because the book takes place before autism was being diagnosed, the family does not understand why Natalie behaves the way she does.
Piper Williams - 12 year old daughter of the warden, has a collection of prisoner baseballs, and goes against her father's rules.
Scout - Moose's best friend and plays baseball with Moose.
Cameron "Cam" Flanagan - Moose's father, works two jobs on Alcatraz (electrician and a guard), looked up to by Moose.
Warden Williams - Father of Piper Williams; is the warden of Alcatraz, very strict about rules.
"Onion" - Prisoner #105 of Alcatraz, loves Natalie Flanagan. He has that nickname by having very greasy, white-like hair.
Annie - Only person on the island who can play ball well.
Jimmy Mattaman - Theresa's older brother, a fly trainer, and a builder of mechanical deices
Theresa Mattaman - Jimmy's younger sister and a tour guide like personality.
Al Capone - Notorious criminal (mob boss and gangster) on Alcatraz, successively accomplishes.
Helen Flanagan- Moose's mother that does everything she can to get Natalie into a school that can help her be 'normal'. However,in doing this, she often overlooks Moose.
Mr. Purdy - The headmaster of the boarding school Natalie went to.
Carrie Kelly - Natalie's teacher to prepare her for Esther P. Marinoff, a boarding school for special needs.