Will is a thirteen-year-old boy nearing the age of mandatory Capping. He possesses a curious mind and quietly questions the oppressive society controlled by the Tripods. Initially hesitant and easily influenced by his peers, he ultimately decides to flee his village to seek a free life in the White Mountains. He learns to rely on others as his dangerous journey forces him to mature.

The Tripods are massive, mechanical, three-legged beings that subjugated humanity a century ago. They control the human population by surgically implanting mind-altering Caps into teenagers. They possess advanced technology, including flexible metal tentacles, tracking devices, and the ability to travel rapidly across land and sea.

Henry is Will's older, physically stronger cousin. Following the death of his mother, he forces his way into Will's escape from Wherton mostly out of self-interest. Though initially combative and prone to jealousy, he proves observant and capable during their dangerous journey, offering critical support when Will is injured.

Jean-Paul is a thin, clever French boy who wears makeshift, mismatched glasses of his own invention. Orphaned and raised by his aunt and uncle, he taught himself English and loves studying old technology. He joins Will and Henry's expedition because he wants to avoid being Capped and losing his analytical mind.

Eloise is the polite, beautiful daughter of a French count and countess. She wears a turban to cover her head, hiding the physical evidence of her recent Capping. She teaches Will French and horseback riding, enjoying a life of luxury that masks her absolute subservience to the Tripods' social order.

Ozymandias is a secret recruiter for the resistance who travels in the guise of a mentally unstable Vagrant. He wears a fake Cap to avoid suspicion while searching for young people willing to flee to the White Mountains. He provides Will with a map, a compass, and the truth about the Tripods.

Jack is Will's older cousin and closest friend at the start of the story. Before his Capping, he openly questions the Tripods and entertains the idea of living as a free Vagrant. Once Capped, his entire personality changes, and he becomes a distant, compliant worker who dismisses his old ideas.

Captain Curtis commands the ship Orion. He is a secret ally of the resistance who risks his life and livelihood to smuggle runaway boys across the English Channel to France.

The Comtesse is a wealthy French noblewoman who rules over a comfortable castle estate. She acts as a generous benefactor to Will, nursing him back to health and even offering to adopt him into the nobility.

The Comte is Eloise's father and the lord of the Red Tower castle. He is brave but socially awkward, enjoying the privileges of nobility while remaining entirely submissive to the Tripods.

Will's father is a stern resident of Wherton who values the few remaining mechanical artifacts from the past. He strictly enforces social rules and demands Will stay away from the Vagrants.

Will's mother is a practical woman in Wherton who takes Henry in after his mother's death, expecting Will to show charity and kindness to his grieving cousin.

Henry's mother is Will's aunt. Her untimely death leaves Henry orphaned and severs his remaining emotional ties to Wherton, precipitating his decision to leave the village.