Nico is the demigod son of Hades who possesses necromantic and shadow-traveling abilities. He struggles with a dark personal aesthetic, deeply held negative self-images, and post-traumatic stress from past events. Despite his harsh exterior and affinity for the Underworld, he cares deeply for his boyfriend and is fiercely determined to rescue those left behind.

Will is the demigod son of Apollo and the primary healer at Camp Half-Blood. He radiates physical and emotional light, preferring optimism and positivity. As he prepares to accompany his boyfriend into the Underworld, Will faces significant anxiety about leaving the sunlit world and entering a place of absolute darkness and death.

Bob is a reformed Titan who previously sacrificed himself by falling into Tartarus to save other demigods. Having had his memories washed away in the River Lethe, he transformed from the cruel Titan Iapetus into the gentle, helpful Bob. He is currently trapped somewhere deep in the Underworld, desperately calling for help.

Nyx is the protogenos, or primordial goddess, of night and absolute darkness. She rules over her domain in Tartarus with a hatred for change and a desire for all beings to accept their darkest forms. She views Nico's presence as a challenge and seeks to force him to embrace pure darkness.

Gorgyra is a pale blue nymph with indigo lips who resides by the River Acheron. She weaves the souls of the lost into her gossamer dress to give them company. Fascinated by human connection, she demands stories of romance and emotion in exchange for the use of her boat.

Percy is a veteran demigod and son of Poseidon. Having previously survived Tartarus alongside Annabeth, he understands the horrific psychological toll of the Underworld and offers solemn advice to Nico and Will before they depart.

Annabeth is a demigod daughter of Athena who survived Tartarus with Percy. She provides practical and emotional advice to the boys, emphasizing that their relationship and happy memories will be their strongest defense against the despair of the Underworld.

Small Bob is a magical spartoi creature in the form of a cat. He is Bob the Titan's fiercely loyal pet who fell into Tartarus alongside his master. He can transform into a large saber-tooth tiger when threatened.

Dionysus, also known as Mr. D, is the god of wine and the director of Camp Half-Blood. Because he has divine expertise in madness and altered states of consciousness, Nico consults him for help managing severe trauma-induced dreams.

Chiron is the centaur activities director at Camp Half-Blood. He officially issues the quest to Nico and Will, helping them pack while expressing concern for the severe dangers of their journey.

Rachel is the mortal Oracle of Delphi at Camp Half-Blood. She channels a formal prophecy that directs Nico to find the voice calling to him and warns that he must leave something of equal value behind.

Epiales is a demon of nightmares and a child of Nyx. The shadowy entity traps victims in agonizing time loops and forces them to relive their worst fears and memories to feed its mother's desire for darkness.

Screech-Bling is a subterranean humanoid frog known as a troglodyte. He and his people reside in the Underworld, and he helps Nico and Will travel through the dark tunnels leading closer to Tartarus.

Hiss-Majesty is a timid troglodyte who joins the group traveling toward Tartarus. He finds sudden bravery and elevated speed when Will gifts him a sunlamp to carry.

Menoetes is an Underworld worker with the head of a red Angus bull and a crimson human torso. He tends to Hades' cattle and nectar groves, demanding a bribe from Persephone's garden in exchange for his silence regarding Nico and Will's presence.

Persephone is the goddess of springtime and queen of the Underworld. She maintains a garden of rare plants in her husband's domain and offers Will profound advice regarding how light and dark must coexist within a relationship.

Amphithemis is a Lamian centaur and a mania, a soul obsessed with an unfulfilled task. Originally a river spirit cursed by Hera, he wanders Tartarus endlessly, aggressively searching for an unknown child.

Bianca is Nico's deceased older sister. She died years prior while serving as a Hunter of Artemis, and her memory is frequently manipulated by Nyx to cause Nico psychological distress.

Maria is Nico's deceased mortal mother. She was killed by Zeus when Nico was a child, a tragedy that severely impacted his life.

Naomi is Will's mortal mother, a famous alt-country singer. Will has fond memories of traveling with her and frequently hums songs she composed to comfort himself in the dark.

Hades is the Greek god of the dead and the Underworld. Though stern, he wishes for his son to find genuine happiness and subtly facilitates the quest by sending Nico the prophecy.

Piper is a demigod friend of Nico who shares his grief over the loss of Jason Grace. She offers Nico a listening ear and shares her own experiences with changing labels and embracing her identity.

Jason is a deceased demigod friend of Nico. Nico was forcibly outed in front of him by Cupid, a traumatic memory that Nyx frequently weaponizes against Nico in his dreams.

Nemesis is the goddess of balance and retribution, and a daughter of Nyx. She acts upon her own motives regarding the cosmic equilibrium of the Underworld.

Hypnos is the god of sleep and a child of Nyx. He possesses immense power over slumber and dreams, observing the demigods' journey through his mother's domain.

Carl is a red-eyed monster covered in matted fur. He and his brother are locked in an absurd, endless cycle of dying and regenerating to collect bones.

Bartholomew is a monster caught in a bizarre rivalry with his brother Carl, eagerly welcoming death just to respawn and collect more bones.

Cupid is the god of love who brutally forced Nico to out himself and confess his crush in front of Jason Grace. This cruel act serves as one of the primary sources of Nico's recurring night terrors.

Echidna is a monstrous chimera. Nico recalls encountering her right before his horrific first fall into Tartarus.

Otis is a giant who previously captured Nico. Alongside his twin, he trapped the demigod inside a bronze jar on Gaea's orders.

Ephialtes is a giant who helped his twin brother Otis capture Nico. They sealed him in a jar with limited oxygen during a past conflict.