Annabeth is the demigod daughter of the Greek goddess Athena and military history professor Frank Chase. She possesses high intelligence and a deep passion for architecture and strategy. Tasked with a difficult solo quest by her mother, she feels the pressure of representing Camp Half-Blood while trying to reunite with her long-absent boyfriend.

Leo is a talented mechanic and the son of Hephaestus and Esperanza Valdez. He constructed the flying warship Argo II and maintains its complex systems. He often feels isolated since he is the only demigod without a romantic partner on the ship. He relies on humor to mask his insecurities and his deep feelings of guilt.

Piper is the daughter of Aphrodite and movie star Tristan Mclean. She possesses the power of charmspeak, allowing her to persuade others using only her voice. She often feels conflicted about her mother's domain of romance, preferring to focus on practical peacekeeping and supporting her friends during battles.

Percy is the son of Poseidon and Sally Jackson. After spending months with amnesia at Camp Jupiter, he has risen to a leadership position among the Romans. He provides a vital bridge between the two camps and is fiercely devoted to protecting his partner, despite his frustration at being unable to join her on her solo mission.

Jason is a son of Jupiter and a former praetor of Camp Jupiter. His background makes him the Roman equivalent to Percy. He is highly disciplined and strives to integrate his strict Roman upbringing with his current allegiance to his Greek friends.

Hazel is a daughter of Pluto who was brought back to life after dying in the 1940s. She is deeply concerned for her missing half-brother. She rides a magical horse named Arion and utilizes her underground sensing abilities to aid the crew on their journey.

Frank is a Roman demigod with the rare ability to shapeshift into animals. He is courageous but holds a deep fear of fire, as his life force is magically tied to a fragile piece of firewood he must protect.

Gaea is the slumbering Earth Mother who seeks to awaken and destroy the Olympian gods. She communicates through the earth and uses possessing spirits, giants, and monsters to sabotage the demigods' journey.

Nico is Hazel's half-brother and a son of Hades. He is a solitary demigod who knows crucial information about the Underworld's layout. The crew races against a tight deadline to find him before he runs out of oxygen in his magical prison.

Reyna is a praetor of Camp Jupiter and the daughter of the Roman war goddess Bellona. She prioritizes the safety and honor of New Rome above all else, commanding respect from her legion while dealing with political pressure from aggressive subordinates.

Octavian is the ambitious augur of Camp Jupiter. He is highly suspicious of the Greeks and continuously looks for reasons to declare war on Camp Half-Blood, using any disruption to advance his political power.

Coach Hedge is a satyr tasked with watching over the younger demigods. He is loud, prone to aggression, and always eager to swing his baseball bat at approaching monsters.

Bacchus is the Roman aspect of the wine god. He is vain, easily bored, and constantly demands entertainment. He refuses to assist the demigods directly unless they prove their combat worthiness in an arena setting.

Ephialtes is a giant in service to Gaea. He acts as a theatrical showman, creating a gruesome underground spectacle designed to publicly execute the demigods and entertain dark forces.

Otis is a giant serving Gaea alongside his brother. He assists in organizing brutal underground games, preparing to fight the demigods with a spear in an arena built for their execution.

Arachne is the legendary mortal weaver who was transformed into a monstrous spider by Athena. She lives in deep underground caverns where she guards a stolen statue and plots the agonizing deaths of any children of Athena who cross her path.

Athena is the Greek goddess of wisdom and strategy. Currently suffering from a split identity due to the conflict between her Greek and Roman forms, she exhibits uncharacteristic rage and demands that her daughter enact harsh vengeance.

Nemesis is the goddess of revenge. She appears to offer a dangerous fortune cookie that will solve a future unfixable problem, warning that true success always requires deep personal sacrifice.

Hercules is the legendary hero who now guards the Pillars of Hercules. Despite his fame, he remains intensely bitter about his past trials and takes out his ancient resentments on the young demigods trying to pass through his territory.

Aphrodite is the goddess of love. She seems unaffected by the Greek-Roman conflict since love is universal. She meets her daughter for tea during a battle, frustrating her with apparent superficiality while providing crucial clues.

Phorcys is an ancient sea god working loosely with Gaea. He masquerades as a human aquarium owner, capturing and drugging mythical sea creatures for terrible aquatic spectacles.

Keto is the goddess of sea monsters. She assists her brother in running a secret, abusive aquarium exhibit meant to entrap young demigods and exploit magical creatures.

Sammy is a mortal boy from New Orleans who forms a surprising historical bridge between two demigods. He carried a deep, misplaced guilt for a friend's tragedy for most of his life.

Achelous is an ancient river god who lost a horn in a legendary battle. He mournfully tries to drown the demigods to prevent his ancient rival from obtaining his second horn and humiliating him further.

Chrysaor is a golden-armored pirate and son of Medusa. Bitter that legends ignore him, he commands a crew of dolphin-warriors and attempts to capture the demigods to sell to his dark allies.

Tiberinus is the Roman river god of the Tiber. Disguised as a vintage movie star, he checks credentials and points a young demigod toward a terrifying underground trial.

Rhea is the mythical mother of Romulus and Remus. She travels with Tiberinus and firmly enforces the ancient rules regarding who can enter sacred underground spaces.

Terminus is the Roman god of boundaries. He strictly enforces the weapon bans of New Rome, causing immediate bureaucratic friction when a heavily armed Greek warship attempts to land.

Ella is a highly intelligent harpy who has memorized ancient prophecies from the Sibylline Books. Her recitation of a specific prophecy provides crucial early clues for a major quest.

Tyson is a friendly Cyclops and a trusted family member. He acts as an ambassador of goodwill between the camps and accompanies a prophetic harpy.

Echo is a nymph cursed to only repeat what others say. She tries to help securing celestial bronze while remaining tragically devoted to a vain hunter.

Narcissus is a famously vain hunter cursed to fall in love with his own reflection. He selfishly hoards the metal needed for ship repairs, forcing others to trick him.

Aphros is an ichthyocentaur (fish-centaur) who runs an underwater training camp for mer-heroes. He detains travelers to ensure they are not threats before generously providing supplies.

Bythos is an ichthyocentaur who helps run the underwater training facility. He provides crucial intelligence about sea monsters and clears safe passage.