The youngest of three children, Luke is an illegal "shadow child" from a rural farm family who must live under a false identity to survive. Overwhelmed and terrified by the large, windowless Hendricks School, he struggles to make sense of the chaotic environment and longs for the safety of his past. He deeply admires his late friend Jen and desperately wishes to find a way to honor her legacy while keeping his true identity secret.

The father of Luke's deceased friend, Jen, and a high-ranking member of the Population Police. Despite his strict, serious demeanor and intimidating job, he secretly works to protect third children from the oppressive government. He secures Luke's enrollment at Hendricks and serves as his sole connection to the outside world, occasionally passing cryptic messages.

A fearless shadow child and Mr. Talbot's daughter who died leading a public rally for the rights of third children. Though she does not appear alive in the present narrative, her memory serves as Luke's primary anchor and moral compass. She was bold and reckless, firmly believing that third children deserved to live openly in society.

A tall, thin boy with dark eyes and an oddly shaped face, who is Luke's abrasive roommate. He initially bullies Luke relentlessly, forcing him to perform humiliating tasks and shouting confusing slang. He claims to be a fellow shadow child hiding under the fake name Scott Renault while going by Jason in the woods, presenting himself as a fearless rule-breaker attempting to organize the other boys.

The wealthy older man who founded the Hendricks School for Boys. He possesses thick white hair and wears expensive clothing while operating from a wheelchair. He established his windowless, maze-like school originally to harbor boys with severe phobias and trauma, keeping himself secluded in a cottage hidden behind dense shrubbery.

A wealthy city boy who died in a skiing accident. His affluent family kept his death a secret so his identity card could be used to save a shadow child. Luke wears Lee's expensive, formal clothes and struggles to convincingly emulate the haughty demeanor expected of a privileged upbringing.

A student at Hendricks who serves as a hall monitor and secretly participates in nighttime meetings outside. After spending thirteen years hiding in a single room, he is deeply terrified of the outside world and struggles with extreme anxiety. He refuses to share his real name, opting for a nickname that simply means the number three.

A history teacher at Hendricks who possesses a firm, authoritative presence in the classroom. He delivers fascinating lectures about historical battles and knights that capture Luke's attention. Unlike the indifferent faculty members, he appears to be one of the few staff members who possesses some awareness of the school's true purpose.

A tall, scrawny student at the Harlow School for Girls with two pale braids. She participates in the clandestine nighttime meetings in the woods alongside the boys from Hendricks, presenting herself as a fellow hidden child seeking to subvert the government's oppressive laws.

A hostile student at Hendricks who is assigned to show Luke around on his first day. He immediately establishes the school's harsh social environment by intimidating Luke, trying to push him down the stairs, and stealing his bag, setting a tone of cruelty for new arrivals.

Luke's two older brothers who grew up legally and openly on their family farm. Though they are not present at the school, Luke frequently reflects on their behavior—such as their sibling rivalry and chess games—to understand the unfamiliar, often hostile social dynamics among the boys at Hendricks.