The false identity assumed by Luke Garner, allowing him to live openly in society while wrestling with the pressures of maintaining the persona of a wealthy city boy.

Jen Talbot's father who works for the Population Police but secretly aids third children, supporting Luke with his new identity and protecting him.

A brave third child whose actions and memory profoundly influence Luke’s journey and decisions.

Luke's bullying roommate at Hendricks who claims to be a third child but holds secrets that challenge Luke’s trust.

A third child who lives in fear of the outside world yet joins a secretive group with Luke, using his nickname because it means "three."

The founder of Hendricks School for Boys, providing a safe haven for children who need it, including those with special conditions.

Luke's history teacher who captivates him with fascinating stories and supports him on his journey at Hendricks.

A student at Harlow School for Girls who participates in a group of third children, later revealing complexities in her role.

A student who sets a challenging tone for Luke's first days at school by bullying him and creating a hostile environment.

Luke's older brothers whose behavior he recalls to navigate social dynamics at Hendricks, offering him a frame of reference.