Percy is a Greek demigod and the son of Poseidon. Stripped of his usual resources and defensive abilities, he must survive a hostile environment that challenges his perception of heroism. He relies on his deep loyalty to his friends and quick thinking to stay alive in an unfamiliar territory.

Annabeth is a Greek demigod and the daughter of Athena. Normally reliant on her intellect and planning, she faces unprecedented challenges in a place where her usual strategies fail. She carries the heavy burden of survival and relies on her deep bond with Percy to maintain hope.

Hazel is a Roman demigod and the daughter of Pluto. She steps up as a leader when the quest members reach a critical intersection early in their flight across Italy. Tasked with learning to manipulate magic, she works to forge her own path rather than accept dark predictions.

Frank is a Roman demigod and the son of Mars. He struggles with his self-worth and the loud, warring voices in his head. Possessing the ability to shape-shift into different animals, he carries a magical piece of firewood that dictates his mortal lifespan.

Leo is a Greek demigod, the son of Hephaestus, and the primary engineer of the Argo II. He often feels like an outsider among his coupled-up friends. He uses his mechanical ingenuity and ability to summon fire to defend the ship and its crew from constant monster attacks.

Jason is a Roman demigod and the son of Jupiter. As a former praetor of Camp Jupiter, he struggles to reconcile his strict Roman upbringing with his growing affinity for his new Greek friends. He acts as a central leader for the crew while wrestling with his shifting identity.

Piper is a Greek demigod and the daughter of Aphrodite. She initially doubts her usefulness in combat compared to her heavily armed peers. She soon learns to utilize her emotional intelligence and charmspeak to protect her friends and uncover the hidden strengths of her heritage.

Nico is a Greek demigod and the son of Hades. He acts as a dark, isolated guide for the Argo II crew, utilizing his connection to spirits to direct them toward the Underworld. He keeps himself physically and emotionally distant, harboring deep personal secrets and a fear of rejection.

Bob is an ancient Titan whose memory was previously wiped, leaving him with a gentle personality and a job as an Underworld janitor. He guides the lost demigods through the hostile abyss, believing them to be his true friends while slowly regaining fragments of his violent past.

Gaea is the primordial earth goddess striving to awaken and destroy the mortal world. She directs her monstrous forces from afar to stall the heroes and controls the primary exit from the Underworld to unleash her armies.

Tartarus is the primordial god of the abyss. He embodies the entire monster underworld and seeks to personally destroy any intruders who attempt to close his doors and stop his monstrous forces from escaping.

Coach Hedge is a combative satyr acting as the adult chaperone for the teenage crew. Known for his aggressive tendencies and love of martial arts, he enthusiastically defends the ship while quietly harboring deep concerns for the family he left behind.

Reyna is a Roman demigod and a praetor of Camp Jupiter. She receives a desperate plea to retrieve a massive statue to prevent a war between the Roman and Greek camps. She risks her leadership position and her life to attempt a peaceful resolution.

Hecate is the goddess of magic and crossroads. She presents the quest members with their required path to the Underworld and personally challenges Hazel to master the Mist to survive the upcoming trials.

Calypso is the immortal daughter of the Titan Atlas, cursed to remain exiled on the island of Ogygia. The gods continually send heroes to her island who are destined to leave, making her bitter toward new arrivals.

Damasen is a giant created specifically to oppose the war god Ares. Unlike his brethren, he is peaceful and was cursed by his parents to live in the abyss, fighting the same beast every day as punishment for refusing to fight the gods.

Khione is the goddess of snow who acts on orders to capture the demigods. She seeks revenge for past humiliations and attempts to freeze the crew, drastically underestimating the heroes in the process.

Akmon is a Kerkopes, a monkey-like thieving creature. He and his brother harass the demigods in Italy, stealing valuable equipment until Leo outsmarts them.

Passalos is one of the Kerkopes brothers. He acts as a highly skilled thief who causes trouble for the crew during their brief stop in Bologna.

Triptolemus is the god of farming. He holds a deep grudge against children of the Underworld but possesses the magical barley the crew requires to survive poison.

Akhlys is the goddess of misery and poison. She controls the Death Mist, a magical shroud the demigods need to hide themselves from roaming monsters.

Nyx is the ancient goddess of night. She commands an army of shadows and resides in a massive dark mansion that stands between the demigods and their exit.