Gregor is an eleven-year-old boy living in New York City who previously rescued his father from the subterranean Underland. He cleans for his neighbor to help his mother pay the bills while his father recovers from illness. Though naturally empathetic and reluctant to fight, he possesses an involuntary, highly lethal combat instinct that makes him a terrifying asset in battle.

Boots is Gregor's two-year-old sister, an innocent and cheerful toddler whose pure-hearted nature contrasts sharply with the dangerous Underland. The giant cockroaches revere her as a princess, and her abduction acts as the catalyst for Gregor's return to the subterranean world. She finds joy in simple things and relies entirely on others for protection.

Ares is a large, powerful bat and Gregor's bonded companion. Still reeling from the betrayal and death of his previous human bond, he exists as a distrusted outcast among his own colony. Despite his initial friction with Gregor over taking orders, he proves to be a fiercely loyal protector who understands the heavy burdens they both carry.

Twitchtip is an irritable, darkly humorous rat operating as an outcast from her own society. She possesses the extraordinary ability to be a "scent seer," allowing her to track anything by smell, including colors and hidden physical traits. She serves as an indispensable tracker for the expedition while enduring severe prejudice from the humans and bats.

Luxa is the fourteen-year-old heir to the throne of Regalia. Highly skilled in combat and diplomacy, she is fiercely independent and protective of her people. Still burdened by the betrayal and loss of her cousin, she harbors a deep-seated hatred for rats but insists on joining the dangerous mission to ensure her kingdom's survival.

Bane is a rare white rat central to the cryptic text that drives the current conflict. According to the Regalians, he represents a terrifying new threat destined to grant the gnawers immense power over the subterranean world. The humans believe Gregor is the only warrior capable of defeating him, setting up a perilous and unavoidable confrontation.

Vikus is a wise, diplomatic leader of Regalia and the patriarch of the royal family. He acts as a primary mentor figure to Gregor, helping him decipher the cryptic Underland prophecies while trying to maintain order among the diverse and often warring species of the subterranean world.

Howard is Luxa's older cousin and a potential heir to the Regalian throne. Hailing from the waterside, he is an expert in seafaring, combat, and water-based healing. Though initially viewed with suspicion by Gregor, his strict adherence to a sacred vow to save anyone in peril at sea makes him a capable and principled ally.

Mareth is a mature, steady, and commanding military specialist in the Underland. Acting as a trainer and coach to Gregor, he brings rational leadership and immense physical strength to the expedition. He serves as a stabilizing, protective force for the younger members of the group.

Nerissa is Luxa's frail, emaciated older cousin who possesses the rare gift of seeing the future. Often dismissed as nervous or unstable by others, she is deeply knowledgeable about the cryptic texts that govern their society and provides crucial insight into Gregor's fated role.

Ripred is a fierce, highly intelligent rat and a formidable, if untrusted, ally to the humans. As a rare fighter with overwhelming combat abilities, he understands Gregor's terrifying battle instincts better than anyone and steps in to teach the young Overlander echolocation and control.

Temp is a giant, sentient cockroach who is entirely devoted to Boots. Seeing the toddler as a revered princess, he is loyal, protective, and gentle, willingly joining dangerous missions despite the lack of respect his species receives from the other Underlanders.

Solovet is Luxa's grandmother and an authoritative military leader in Regalia. Pragmatic and battle-hardened, she believes strongly in preparing Gregor with the necessary weapons and skills to defeat the prophesied enemy by any means necessary.

Stellovet is Luxa's cousin from the waterside. Known for her somewhat cruel demeanor, she actively taunts Luxa about past family tragedies and treason, prompting Gregor to step in and defend his friend.

Pandora is a large bat bonded to Howard. Enthusiastic and adventurous, she plays a vital role in assisting the group's naval journey across the underground Waterway before encountering severe danger.

Aurora is Luxa's bonded bat. Loyal and sturdy, she serves as Luxa's primary mode of transport and companion in battle, quietly supporting the young royal's rebellious decisions even when they break military protocol.

Andromeda is a steadfast Underland bat assigned to assist Mareth and the group on their dangerous journey. She provides critical aerial support when the ships encounter massive aquatic hazards.

Photos Glow-Glow is a giant, sentient firefly enlisted to provide light over the dark underground ocean. Gluttonous and prone to bickering, his constant noise irritates the other members of the party.

Zap is a massive firefly who travels with Photos Glow-Glow. Often complaining and demanding food rations, she offers necessary light in the pitch-black tunnels but heavily tests the patience of her employers.

Goldshard is a fierce golden rat encountered deep within the gnawer tunnels. She is found in a deadly dispute with a military general, fighting savagely for her own mysterious reasons.

Snare is a military general among the gnawers. He is encountered deep in the Labyrinth, engaged in a savage and ultimately fatal conflict over differing loyalties within the rat ranks.

Lizzie is Gregor's intelligent and anxious middle sister who remains in the Overland apartment. Her safety and happiness are a constant motivator for Gregor while he is trapped in the subterranean world.

Mrs. Cormaci is Gregor's kindhearted neighbor in New York City. She actively tries to support his struggling family by hiring him for simple cleaning tasks in exchange for food and money.

Henry is Luxa and Nerissa's deceased cousin. His past betrayal of the humans to align with the rats deeply scars the current characters, leaving Ares an outcast and Luxa burdened with profound grief and anger.