Jonathan is a twelve-year-old boy sent to a harsh, remote island reformatory. He arrives carrying immense guilt over a past tragedy involving his younger sister, Sophia. When the boys are suddenly left to fend for themselves, Jonathan uses his resourcefulness and tactical thinking to suggest they maintain a cover story. He prefers to keep his own past hidden while investigating the strange, crumbling structure of the island.

Colin is a small, sensitive boy who speaks with a pronounced lisp. He claims to be a kleptomaniac, which resulted in his placement at the reformatory. Despite his self-proclaimed thievery, he acts as the conscience of the group, advocating for fairness and democratic processes like voting. He bravely opposes cruel punishments and bullying behavior from the older boys.

The Admiral is the buffoonish but cruel head of Slabhenge Reformatory. He sports bushy eyebrows and speaks in a deep, breathy voice. Believing that society is entirely too soft on juvenile delinquents, he enforces total obedience through physical punishment and severe food restriction. He regularly indulges in chocolates and brandy while depriving the boys of basic necessities.

Sebastian is the aggressive resident bully at Slabhenge. He is older than many of the other boys and quickly recognizes opportunities to seize power. When the facility's leadership dynamic suddenly shifts, he claims the Admiral's office, hat, and sword for himself. He uses charisma and intimidation to convince the others to follow his commands.

The librarian is a very old man with long, thin white hair that drapes over his shoulders. He lives entirely hidden within the reformatory's forgotten library, avoiding the Admiral and the regular staff. Born on the island when it was a psychiatric hospital, he has never left and prefers the company of books and his pet rat. He provides the boys with literature that mirrors their survival situation.

Benny is an opportunistic boy who acts as Sebastian Mortimer's primary henchman. He sneaks into the facility's administrative office to read the other boys' files. He uses this stolen information to blackmail anyone who questions Sebastian's authority.

Walter is a quiet boy who occupies the cell immediately next to Jonathan's. Sent to Slabhenge after stealing a purse to give his mother a gift, he understands the harsh realities of the reformatory. He explains the daily routines and helps Jonathan adjust to the miserable conditions.

Mr. Warwick is the Admiral's obedient assistant. He carries out the Admiral's petty orders, such as denying Jonathan a pillow for his bed. He also warns the boys about a monster supposedly living behind the dark, roped-off staircase known as the Hatch.

Mr. Mongley serves as an acting warden at the reformatory. He enforces the facility's draconian rules with immediate, physical consequences. He demonstrates the staff's cruelty early on by throwing a bucket of cold water on Jonathan merely for whispering.

Mr. Vander is a tall, imposing adult staff member at Slabhenge. He typically oversees the reception of mail and supplies from the delivery boat. He rarely speaks, preferring to stand in the dark while the boys perform the manual labor of unloading the cargo.

Patrick is the young man responsible for piloting the supply and mail boat to Slabhenge. Unaware of the internal dynamics of the reformatory, he simply drops off letters and food. The boys view his regular visits as both a risk to their independence and their only connection to the outside world.

Cyrus is a salty, old boat pilot missing most of his teeth. He transports new arrivals to the reformatory and introduces them to their dismal new home. He possesses a deep knowledge of the local geography and warns that the rising sea is slowly destroying the island.

Jason is one of the youths sent to Slabhenge, specifically for stealing cars. Desperate to escape the island's deteriorating situation, he attempts to write a secret plea for help on the back of an envelope. His actions quickly make him a target of the facility's new leadership.

Gerald is an older boy at the reformatory who participates in the group's schemes to maintain their independence. Because of his size, he is chosen to wear an adult's coat and hat to trick the mail carrier into believing the staff is still supervising the facility.

Sophia is Jonathan's younger sister. She is the central figure in a family tragedy that resulted in Jonathan's placement at Slabhenge. Her memory heavily influences Jonathan's actions, filling him with a paralyzing sense of guilt and driving his desire to protect the other boys.

David is a boy of Japanese descent sent to the reformatory for getting into a severe physical altercation. Despite his violent past, he remains loyal to Jonathan when the rest of the group begins to turn against him.

Miguel is a resident of Slabhenge who was sent away for chronically skipping school. He participates in the group conversations about their pasts and questions Jonathan about his closely guarded secrets.

Tony is one of the youths detained at the reformatory. He shows early skepticism toward the shifting power dynamics when the adults disappear, specifically challenging the idea that the oldest boy should automatically be in charge.

Gregory is a resident of Slabhenge who follows the orders of the newly established leadership. He assists with the physical labor required to keep the building functioning during the massive storm.

James is a small boy who takes advantage of the lack of adult supervision to eat chocolates previously reserved for the Admiral. His minor infraction results in severe consequences when the new leadership decides to enforce discipline.

Ninety-Nine is a massive, surprisingly friendly rat living in the depths of Slabhenge. He serves as the sole companion to the isolated librarian. The rat is accustomed to humans and rests comfortably on his caretaker's shoulder.