Jason is a Roman demigod and the son of Jupiter. He wields control over lightning and the wind. Raised in the strict traditions of Camp Jupiter, he struggles to reconcile his Roman upbringing with his growing attachment to the Greek demigods at Camp Half-Blood. He acts as a key warrior and leader aboard the flying ship Argo II, trying to find a path that honors both sides of his heritage.

Reyna is a Roman praetor and the daughter of Bellona, the Roman goddess of war. She carries the heavy burden of leadership at Camp Jupiter and volunteers for a dangerous cross-Atlantic quest to return the sacred Athena Parthenos to Camp Half-Blood. She holds strict standards for herself, fiercely protects those under her command, and carries hidden guilt regarding her family's past.

Leo is a Greek demigod and the son of Hephaestus. He serves as the primary mechanic and pilot of the flying ship Argo II. Often using humor to mask his insecurities and feelings of isolation, he possesses a rare ability to summon and control fire. He dedicates his mechanical skills to finding a way back to the island of Ogygia to rescue Calypso, while simultaneously trying to stop Gaea.

Nico is a Greek demigod and the son of Hades. His ability to control shadows, summon spirits, and communicate with the dead causes other campers to fear him, leaving him deeply isolated. He volunteers for the taxing mission of transporting the Athena Parthenos across the world, risking his physical form and sanity to keep his friends safe.

Piper is a Greek demigod and the daughter of Aphrodite. She wields the power of charmspeak, an ability that allows her to influence others through her voice. She draws on both her mother's divine gifts and her Cherokee father's teachings to face her fears and support her fellow demigods on their mission to Athens.

Percy is a powerful Greek demigod and the son of Poseidon. He possesses control over water and a fiercely loyal disposition toward his friends. He fights alongside the other chosen demigods on the Argo II, often rushing to the front lines of battle without considering his own safety.

Annabeth is a Greek demigod and the daughter of Athena. She relies on her sharp intellect, tactical planning, and logic to solve problems during the quest. Her strategic mind sometimes struggles in situations that require emotional intuition rather than pure reason.

Frank is a Roman demigod and the son of Mars. He holds the ability to shape-shift into various animals, a power inherited from his family lineage. He serves as an essential warrior on the Argo II, providing brute strength and tactical knowledge to the crew.

Hazel is a Roman demigod and the daughter of Pluto. She commands powers over the earth and precious metals, and possesses the ability to manipulate the magical veil known as the Mist. She supports the crew on their journey to Athens with her unique magical capabilities.

Octavian is the Roman augur and a descendant of Apollo. He seizes power at Camp Jupiter in Reyna's absence and aggressively pushes the Roman legion to attack Camp Half-Blood. He believes he is destined to save Rome by completely destroying the Greek demigods.

Gaea is the primordial earth goddess and the central enemy of the demigods. She intends to wake from her deep slumber by shedding the blood of two demigods at the Acropolis in Athens. Her awakening threatens to overthrow the Olympian gods and destroy mortal civilization.

Coach Gleeson Hedge is a satyr and the aggressive, baseball-bat-wielding chaperone for the demigods. Despite his combative exterior, he cares deeply for his traveling companions and constantly worries about his pregnant wife waiting for him at Camp Half-Blood.

Hylla is the queen of the Amazons and the older sister of Reyna. She leads her fierce warriors in an alliance with the Hunters of Artemis to track down and eliminate Gaea's monsters across North America.

Calypso is an immortal figure trapped on the island of Ogygia. She possesses a beautiful singing voice and awaits the fulfillment of a promise made by Leo, who vowed to return and rescue her from her isolation.

Will is a Greek demigod and the son of Apollo. He works as a dedicated healer in the camp's infirmary. He refuses to be intimidated by underworld powers, often enforcing strict medical advice on stubborn patients like Nico.

Orion is a giant and a deadly hunter revived by Gaea to pursue the Athena Parthenos. He holds a deep hatred for the Amazons and the Hunters of Artemis, specifically targeting strong female leaders.

Apollo is the god of music, poetry, and prophecy. He currently suffers the wrath of Zeus, who blames him for the escalating war with Gaea. He strikes bargains with the demigods to aid their quest in hopes of winning back his father's favor.

Bryce is a cruel Roman legionnaire and a legacy of Orcus. Banished for insubordination but reinstated by Octavian, he possesses the ability to command undead forces and relishes inflicting pain on others.

Kymopoleia is the goddess of violent sea storms and a daughter of Poseidon. Angered by her lack of recognition and her arranged marriage, she initially aligns with Gaea's forces before entertaining offers from the demigods.

Asclepius is the god of medicine, closely guarded by statues and mechanical defenses to prevent him from altering mortal fates. He possesses the unique knowledge required to brew a potion capable of curing death itself.

Nike is the goddess of victory, split painfully between her Greek form and her Roman aspect, Victoria. Her intense competitive aura forces the demigods to fight one another until they can subdue her and extract valuable information about their quest.

Kekrops is the part-man, part-snake original founder and king of Athens. Harboring an ancient grudge against Athena, he lives in underground tunnels and strikes a treacherous deal with the earth mother to destroy the demigods.