Gregor is an eleven-year-old boy who falls through a grate in his laundry room into the subterranean Underland. He steps into the role of protector for his baby sister and trusts his intuition over pure logic. Though the Underlanders believe him to be the warrior of a mysterious prophecy, he prefers finding peaceful solutions over raising a weapon.

Boots, whose real name is Margaret, is Gregor's two-year-old sister. She is joyful, fearless, and naturally diplomatic. She immediately befriends the giant insects, bats, and humans of the Underland without prejudice, treating all creatures as equals.

Luxa is the fourteen-year-old future queen of Regalia, the human stronghold in the Underland. She is a highly skilled bat rider with a proud demeanor that masks her anxiety for her kingdom's survival. Having lost her parents to rats two years earlier, she harbors strong resentment toward the gnawers.

Henry is Luxa's sixteen-year-old cousin and a member of the Underland royalty. He carries himself with a mix of bravery and arrogance. He believes strongly that survival depends on aligning with the powerful and regularly mocks creatures he perceives as weak.

Vikus is a sage and counselor in the city of Regalia. He acts as a mentor figure who works tirelessly for peace among the varying species of the subterranean world. He guides the questers with patience and possesses a willingness to trust those others might reject.

Ripred is a highly intelligent, battle-scarred rat with a diagonal mark across his face. Unlike most of his species who blindly follow their king, he operates by his own moral code and possesses wisdom unique among the gnawers. He has experience above ground and reads English.

King Gorger is the giant leader of the rats in the Underland. He wears a golden crown on one ear that previously belonged to Luxa's parents. He seeks to destroy all humans in Regalia and views the prophecy as a direct threat to his reign.

Solovet is Vikus's wife and a commanding presence in Regalia. Though she speaks gently, she oversees the Underland human army and forms battle tactics. She is also an expert healer with extensive knowledge of survival medicine.

Gregor's unnamed father is a scientist from New York City who vanished two years and seven months before the events of the story. His disappearance deeply wounded his family. The rats hold him captive, forcing him to design weapons for their war effort.

Ares is a large, powerful bat from the Underland who participates in the quest. He is exceptionally strong and capable in the air, serving as a vital companion on the dangerous journey through the lightless tunnels.

Temp is a giant cockroach who discovers the Overlanders. He forms an immediate attachment to Boots, recognizing her joyful nature as something sacred to his species. He provides transport and protection for the toddler.

Tick is a giant cockroach who joins the quest primarily due to her deep reverence for Boots. She acts as a silent but dedicated guardian for the young girl as the group moves through the dangerous, cramped tunnels.

Mareth is a loyal and capable guard in the city of Regalia. He helps look after the young Overlanders during their initial days in the palace and readily fights to defend the kingdom from sudden attacks.

Perdita is a strong guard of Regalia who protects the human royals and their guests. She is willing to risk her own safety against dangerous predators to ensure the Overlanders survive.

Dulcet is a kind servant in the palace at Regalia. She cares for Boots with great patience, providing the toddler with baths, clean clothes, and gentle attention while keeping Gregor informed about palace operations.

Aurora is a golden bat who serves as Luxa's mount. She is bound by a sacred vow of loyalty to her rider, ensuring they fight to the death for one another in the dark skies beneath the earth.

Euripides is a large, capable bat who serves as a trusted mount to the elder diplomat of Regalia. He provides transport and quiet support during important negotiations.

Treflex is a small brown spider who lives in the Underland. He becomes caught up in the shifting alliances of the subterranean war when the rats suddenly turn against his kind.

Gox is a large orange spider who joins the diverse group of creatures assembled for the prophecy. Like other spinners, she operates on a strict, unsentimental code of survival that heavily contrasts with human emotions.

Fangor is a towering, six-foot-tall rat with enormous incisors. He prowls the borders of the human territory with his partner, eager to hunt any Overlanders who slip past the palace defenses.

Shed is a vicious rat who hunts the dark riverbanks. He possesses a cruel sense of amusement regarding his prey and harbors a deep hatred for the Overlanders.

Nerissa is a royal Underland teenager with a special gift for psychic knowing. She experiences intense dreams and premonitions about the future, allowing her to anticipate the needs of others.

Athena is the Queen of the bats in the Underland. She holds the authority to decide whether her species will participate in the human quests and requires definitive proof of the Overlander's identity.

Wevox is the Queen of the Spiders, known as spinners. She commands the webs and decides the fate of trespassers who blunder into her domain, prioritizing the safety of her colony above old treaties.

Lizzie is Gregor's younger sister who remains in New York City. While she does not fall into the Underland, her presence in Gregor's thoughts reminds him of the family he left behind and his responsibility to return home.