Banished from Mount Olympus by his father Jupiter, the former god of music and archery is currently trapped in the body of a mortal teenager named Lester Papadopoulos. He is physically vulnerable, easily fatigued, and suffering from an infected ghoul scratch that threatens to turn him into a zombie. Plagued by guilt over the cruelty he casually inflicted during his divine existence, he slowly develops a mortal conscience while trying to save the Roman demigod camp.

A twelve-year-old demigod daughter of Demeter who holds the magical contract commanding Apollo's servitude. She is fiercely protective of her friends, a trait driven by her intense fear of abandonment following her father's murder and Jason's death. She wears distinct clothing, commands formidable plant-based magic, and is willing to recklessly throw herself into combat to protect Apollo.

A praetor of the Twelfth Legion who shoulders the heavy responsibility of leading a decimated camp. He carries a piece of firewood physically tied to his mortality due to a curse from Hera; if the wood burns away, he will die. Despite this looming fatalism, he leads with quiet, sincere strength and does not shy away from sharing his vulnerabilities with his troops.

The senior praetor of Camp Jupiter and a daughter of Bellona. She has carried the strategic and emotional weight of the Roman legion for years, leaving her profoundly fatigued. She privately struggles with a prophecy delivered by Venus stating that no demigod or mortal will ever heal her heart, a decree that isolates her emotionally from those around her.

A centurion of the Fifth Cohort and a daughter of Pluto. She is exceptionally mature for her age and handles her duties with steady precision. Her mastery over the earth and underground spaces makes her an indispensable guide when the heroes must navigate ancient, subterranean tombs filled with traps and undead guards.

The seventh and final king of ancient Rome, also known as Tarquin the Proud. He has been resurrected as a powerful, gas-animated skeleton who commands an army of zombified Roman legionnaires and ghouls. Operating from a subterranean lair beneath a Tilden Park carousel, he aims to exploit the imminent attack on Camp Jupiter to conquer New Rome from the sewers.

A notoriously cruel, resurrected Roman emperor and a leading member of the Triumvirate. He is responsible for the recent murder of Jason Grace. Viewing war as a form of grand entertainment, he commands a heavily armed fleet of yachts equipped with Greek fire mortars and exhibits a thoroughly modern, decadent approach to villainy.

A vain, resurrected Roman emperor who forms part of the villainous Triumvirate. Blinded during his previous clash with Apollo in Indianapolis, his lack of sight has only fueled his vindictive rage. He shares a complicated ancient history with Apollo, having once been the god's lover before their relationship ended in tragedy.

A pink-haired, gum-chewing Roman legionnaire and daughter of Terpsichore. She possesses high nervous energy and a blatant disregard for traditional Roman military discipline, frequently skipping sentry duty to socialize with nature spirits. Despite her casual attitude, she has an unparalleled knowledge of the secret paths surrounding the camp and a talent for deciphering cryptic clues.

A heroic Roman demigod who perished in a previous clash with Caligula. Though he is deceased throughout the novel, his sacrifice heavily influences the actions of the remaining characters. He leaves behind architectural dioramas detailing his wish to build shrines for every obscure deity, a final request that deeply moves the Roman legionnaires.

A harpy gifted with a photographic memory. She is currently tasked with reconstructing the lost Sibylline Books of prophecy. Because she needs living skin as a medium to keep the prophetic texts organized, she carefully dictates the words to be tattooed onto her companion, Tyson.

A gentle Cyclops currently residing in New Rome. He endures the painful process of having the reconstructed Sibylline Books tattooed onto his back by Ella, acting as a living canvas to preserve the ancient warnings necessary for the camp's survival.

An obscure god of silence who has been imprisoned by the emperors. They use his amplified power to enforce a total magical communications blackout over Camp Jupiter. During Apollo's divine days, he relentlessly bullied and humiliated the silent god, creating deep-seated resentment that Apollo must now face to secure his help.

An ancient prophetic figure who spurned Apollo's romantic advances millennia ago. In retaliation, Apollo cursed her to live eternally without the benefit of youth. Now reduced to a fading life force contained within a glass jar, she is imprisoned alongside Harpocrates at Sutro Tower.

A local faun who frequents the areas around Camp Jupiter. He is closely connected to the region's nature spirits and dryads. When the camp comes under threat, he assists Lavinia in an unauthorized attempt to organize the woodland creatures against the invading fleet.