Percy is a Greek demigod and the son of Poseidon. He arrives at Camp Jupiter with no memories other than his name and the fact that he has a girlfriend named Annabeth. Armed with a strong moral compass and water-manipulating abilities, he joins the Roman camp's Fifth Cohort. He frequently relies on his instincts to assess dangerous situations since his past remains clouded.

Hazel is a 13-year-old daughter of Pluto and a gris-gris queen from New Orleans. She belongs to the Fifth Cohort at Camp Jupiter and harbors secrets about her life in the 1940s. A curse causes precious gems and metals to spontaneously emerge from the earth around her, bringing bad luck to anyone who takes them. She struggles with guilt regarding her past actions and deeply values her current friendships.

Frank is a 16-year-old Canadian camper sent to Camp Jupiter by his grandmother following his mother's military death. He feels clumsy and unsure of his abilities within the camp. His life is tied to a specific piece of firewood that will end his life if it burns completely. He relies on archery but struggles to understand the ancient legacy his family holds.

Nico is the son of Pluto and Hazel's half-brother. He spends his time investigating the Underworld and gathering information about the impending war. He is secretive and quiet, keeping most of his knowledge to himself while checking in on his sister.

Reyna is the praetor of the Twelfth Legion at Camp Jupiter and a daughter of the Roman war goddess Bellona. She works to maintain order in the camp following the disappearance of her fellow praetor, Jason Grace. She is a strict but pragmatic leader who uses lie-detecting greyhounds to evaluate newcomers.

Juno is the Roman goddess of marriage and the counterpart to the Greek Hera. She orchestrates Percy's arrival at Camp Jupiter to force the Roman and Greek demigods to interact. She issues strict warnings to Percy about the misery he will face if he proceeds.

Mars is the Roman god of war and the counterpart to Ares. He visits Camp Jupiter to inform the legion that Thanatos has been captured. Unlike his chaotic Greek counterpart, Mars demonstrates a gruff but genuine interest in helping his son understand his family's military legacy.

Gaea is the ancient earth goddess and the mother of giants and monsters. She seeks to overthrow the Olympian gods by waking her giant children and destroying the demigod camps. She communicates with the characters through their dreams and the earth itself.

Octavian is the camp augur who reads the future by gutting stuffed animals. He is highly ambitious and hopes to be elected praetor to replace Jason Grace. He deeply distrusts outsiders and opposes the quest to Alaska.

Alcyoneus is a giant son of Gaea born specifically to oppose Pluto. He operates from Hubbard Glacier in Alaska, where he holds the god of death captive. He cannot be killed within his home territory of Alaska.

Polybotes is a giant and another son of Gaea. He commands an army of cyclopes, centaurs, and basilisks marching south to destroy the Roman demigod camp.

Tyson is a cyclops and the son of Poseidon. He searches the country for Percy, following his trail with the help of a hellhound named Mrs. O'Leary. He maintains a cheerful disposition despite his immense size and strength.

Annabeth is a demigod from Percy's past life. She remains a constant fixture in his fragmented memories, driving his desire to understand who he is and where he came from.

Thanatos is the god of death who ensures souls remain in the Underworld. His capture has left the Doors of Death open, allowing monsters and enemies of the gods to return to life continuously.

Marie is Hazel's mother and a former gris-gris queen from New Orleans. She allowed her desire for wealth to overshadow her daughter's safety, eventually moving them to Alaska under the influence of an ancient goddess.

Captain Emily Zhang is Frank's mother, a Canadian military soldier who died protecting her unit. She shielded Frank from the realities of his godly parentage, preferring to raise him safely before her deployment.

Frank's grandmother resides in an old family estate in Canada. She is a strict guardian who passes down the Zhang family history and legacy, urging her grandson to embrace his unique abilities.

Pluto is the Roman god of the Underworld and riches. He visits Hazel briefly, attempting to warn Marie away from a dangerous path, but remains largely absent from his children's daily lives.

Phineas is a blind seer resurrected by Gaea from the Underworld. He uses his prophetic abilities to extort information, carrying a bitter and selfish demeanor as he torments the starving harpies around him.

Ella is a red-feathered harpy with a photographic memory. She remembers everything she reads, making her a valuable target for those seeking ancient prophecies and information.

Hylla is the queen of the Amazons and Reyna's older sister. She operates her empire from a corporate headquarters while fighting nightly battles against an undead challenger to maintain her throne.

Otrera is the first queen of the Amazons, brought back from the Underworld to seize control of the faction and keep them from aiding the Roman camp.

Arion is an incredibly fast, magical horse with a rough temperament. He eats precious metals and transports Hazel and her friends at sonic speeds across the continent.

Iris is the rainbow and messenger goddess who runs an organic shop. She helps Frank understand his family legacy by showing him visions from his past.

Fleecy is a cloud nymph who assists Iris at her store. She helps process magical messages for demigods and prepares restorative teas.

Sammy is a boy from Hazel's past who works at a horse stable. He was Hazel's closest companion before she was forced to move to Alaska.

Leo is a Greek demigod traveling on the flying warship Argo II. He sends a message to Camp Jupiter requesting a peaceful landing.

Lupa is the immortal wolf goddess who trains Roman demigods. She prepared Percy for his journey south while he was suffering from amnesia.

Jason Grace is the missing praetor of the Twelfth Legion. His absence creates a power vacuum at Camp Jupiter, prompting the upcoming elections.

Terminus is the Roman god of boundaries who appears as a limbless stone statue. He strictly enforces the rules of the camp, specifically the ban on carrying weapons across the city lines.

Gwen is a centurion of the Fifth Cohort. She is fatally wounded during a training battle but inexplicably returns to life due to the open Doors of Death.

Dakota is a centurion of the Fifth Cohort. He helps direct the outcast group of legionnaires alongside Gwen.

Kinzie is a security guard for the Amazons. She is a highly trained warrior who guards the entrance to their Seattle headquarters.

Mrs. O'Leary is a massive hellhound from the Greek demigod world. She travels with Tyson as they track Percy's scent across the country.