Talon is a fourteen-year-old Downsider who serves as a skilled Catcher. He wears clothes stitched from discarded surface fabrics and fastens a watch to his ankle to show his disregard for conventional time. Naturally curious about the world above, he questions the strict Downside doctrines that demand total separation from surface dwellers. His desire to procure medicine for his sick sibling forces him to interact directly with the forbidden Topside.

Lindsay is a fourteen-year-old girl who recently moved from Texas to New York City to live with her father. With her mother away in Africa studying white rhinos, Lindsay feels entirely out of place in her new environment. She struggles to connect with her preoccupied dad and her openly hostile stepbrother. Resourceful and observant, she has self-defense training and prefers reading classic science fiction over attending teen parties.

Railborn is Talon's closest friend and a strict adherent to Downside law. He prefers Hunting to Catching and views the Topsiders as a definitive threat to his underground society. He harbors active ambitions for leadership and explicitly seeks the approval of the corrupt Wise Advisors. He possesses a strong sense of duty that frequently clashes with Talon's rebellious curiosity.

Mark is Lindsay's father and a highly driven civil engineer. He focuses intensely on constructing a massive aqueduct intended to supply New York City with drinking water for centuries. His obsession with his career leaves him little time to connect with his daughter. He views his massive infrastructure project as his legacy.

Gutta is a fourteen-year-old Downsider who patrols the tunnels alongside Talon and Railborn. She serves as a calming presence within their small group and clearly favors Talon's natural leadership over Railborn's ambition. She values loyalty and follows the group's activities closely. Her pragmatic approach helps ground the boys during their underground rotations.

Alfred Ely Beach is a historical engineer who secretly built New York City's first pneumatic subway system to bypass corrupt city politicians. He possesses a reputation for eccentricity and visionary design. His subterranean creations form the foundation of the underground world's mythology.

The Champ is an older man who lives in an abandoned municipal pool near FDR Drive. He exists on the periphery of both Topside and Downside societies without claiming allegiance to either. He is a Pearl Harbor veteran with a white beard and a highly organized living space. He acts as an independent observer and a father figure for Talon, offering practical medical advice and necessary supplies.

Becky is a highly talkative student at Icarus Academy who forcefully befriends Lindsay. She speaks constantly and often provides unwanted commentary on school social dynamics. Despite her overwhelming conversational style, she proves useful when research tasks require library access.

Todd is Lindsay's stepbrother, living under Mark Matthias's guardianship after his mother joined a cult. He possesses a handsome face but an incredibly abrasive personality. He frequently antagonizes Lindsay and acts with cruelty toward anyone he considers beneath his social standing. He takes unearned credit for protecting the household during a break-in.

Robert is a nineteen-year-old unhoused man who struggles with despair on the streets of New York. Searching for warmth and an escape from his bleak surface existence, he wanders into the subway tunnels. He demonstrates a willingness to abandon his past entirely for a chance at a new underground life.

Pidge is Talon's younger sister who lives in the subterranean Downside community. She relies on a battered, battery-operated toy puppy for comfort while recovering from a serious illness. She eagerly listens to her older brother's stories about the strange people who live on the surface.

Skeet is Railborn's father and an active adult member of the Downside community. He takes a practical interest in the boys' rotational duties and expects them to follow standard underground procedures. He represents the traditional adult expectations within their isolated society.