Percy is a fourteen-year-old demigod son of Poseidon. Known for his sarcastic humor and impulsive bravery, he tries to balance his dangerous mythological life with normal teenager problems. His fatal flaw is excessive loyalty, meaning he willingly risks his own safety to protect his friends and innocent bystanders. He controls water and communicates telepathically with equine creatures.

Annabeth is a highly strategic demigod daughter of Athena. She lives at Camp Half-Blood year-round and holds a long-standing desire to lead an official quest. Though fiercely intelligent, she suffers from hubris and becomes extremely frustrated when situations defy logical reasoning. She harbors a strong protective instinct toward Percy and struggles with complicated feelings regarding her former friend's betrayal.

Grover is a satyr and Percy’s dedicated best friend. As an official searcher, his life’s purpose involves tracking down the lost god of the wild. He faces extreme pressure from the elders, who threaten to revoke his license if he fails to produce results. Despite his natural anxiety and intense fear of underground spaces, he pushes himself into dangerous situations to fulfill his duty.

Tyson is a cyclops and Percy’s half-brother. Despite his towering size and immense physical strength, he maintains a childlike innocence and a positive outlook. He has a natural affinity for machinery and working in the underwater forges. He often acts as a peacemaker and approaches legendary figures with starry-eyed admiration.

Nico is the eleven-year-old demigod son of Hades. Following a deep personal tragedy, he isolates himself from Camp Half-Blood, wearing a battered aviator jacket and carrying intense anger. He struggles with feelings of alienation and channels his grief into a dangerous obsession with the Underworld. His primary motivation involves finding a way to reverse his sister's untimely demise.

This antagonistic force represents the union between a bitter demigod son of Hermes and the Titan Lord of Time. Luke channels his anger toward the Olympian gods to rally monsters and unclaimed demigods to his cause. Together, they plot an invasion of Camp Half-Blood, blending ancient divine malice with modern demigod resentment.

Rachel is a mortal teenager who possesses the rare ability to see through the magical Mist that hides monsters. She comes from a wealthy family enriched by real estate development, a background she views with deep discomfort. She is blunt and observant, and she bravely assists the demigods by acting as a clear-sighted guide through confusing magical terrain.

The legendary architect of the Labyrinth and the newly appointed swordmaster at Camp Half-Blood. He carries intense emotional baggage from a long history of family tragedy and divine curses. Bitter and haunted by past mistakes, he operates with ambiguous motives while maintaining an unparalleled understanding of complex mechanics and mazes.

Minos is a cruel ancient king who now serves as a ghostly judge in the Underworld. He acts as a spiritual advisor to young demigods, manipulating their grief for his own selfish motives. He holds a permanent grudge against the inventor who humiliated him centuries ago and ruthlessly pursues revenge from beyond the grave.

The Olympian god of the forge, blacksmiths, and fire. He appears as a massive mechanic with physical injuries resulting from his rejection by his family. Cynical about interpersonal relationships, he prefers the predictability of machines and advises others to trust only what they build with their own hands.

The Queen of Olympus and goddess of marriage. She presents herself as a nurturing, motherly figure who wishes to sponsor the demigods' quest to maintain family unity. However, her pleasant demeanor masks a ruthless streak and intense hypocrisy, as she deals harshly with those who reject her vision of a perfect family.

The lost god of the wild. He is an ancient deity whose existence is sustained only by the desperate faith of the satyrs who search for him. As the natural world shrinks, his power wanes, reducing him to a lingering presence in an isolated sanctuary.

Percy's mortal mother. She possesses the ability to see through the Mist and understands the dangers her son faces. She provides a warm, stable home environment in New York City and fully supports Percy's dual life in the mythological world.

An English teacher at Goode High School and Sally Jackson's boyfriend. He is a supportive, ordinary mortal who remains unaware of the mythological chaos surrounding Percy's life, offering conventional paternal advice and stability.

The wise centaur who serves as the activities director at Camp Half-Blood. He acts as a stabilizing mentor figure for the demigods, organizing the camp's defenses and offering historical context for their struggles against the Titan army.

A tough demigod daughter of Ares. She initially researches the underground maze with Annabeth but vehemently refuses to return after experiencing its psychological tricks, which demonstrates the severity of the threat facing the campers.

A tree nymph living at Camp Half-Blood. She deeply cares for her satyr boyfriend and worries constantly about his safety when he descends into underground environments that separate him from nature.

Nico's deceased older sister. She resides in the Underworld and acts as a benevolent spirit, urging her brother to abandon his dangerous grudges and accept her passing rather than attempting to reverse the natural order of life and death.

A legendary Hundred-Handed One from ancient myths. Imprisoned for ages in Alcatraz, he has lost his self-confidence and grown depressed, appearing physically diminished and entirely lacking the mountain-shattering strength of his reputation.

A fearsome, dragon-like ancient monster who serves as a jailor for Kronos. Armed with poisoned swords, she takes sadistic pleasure in tormenting her prisoners and ruthlessly pursues anyone who attempts to escape her custody.

A greedy, three-bodied monster who operates the Triple G Ranch. He views sacred and endangered animals strictly as sources of profit, happily selling goods to the Titan army and openly breaking deals if it serves his financial interests.

A son of Ares who works as a cowherd at the Triple G Ranch. He initially follows his boss's cruel orders out of apathy but harbors a growing discomfort with the unethical treatment of the animals under his care.

The two-faced minor god of choices, doorways, and beginnings. He delights in cornering heroes in confusing corridors and forcing them to make high-stakes, stressful decisions with potentially deadly consequences.

The daughter of the Titan Atlas, exiled to the phantom island of Ogygia. She acts as a gentle, healing figure who tends to injured heroes that wash ashore, though her isolated existence forces her to endure perpetual heartbreak when they inevitably leave.

An unclaimed demigod son of Nemesis. Abandoned by the Olympians, he carries deep-seated bitterness and seeks to prove himself in gladiatorial combat, channeling his anger into loyalty for the Titan cause.

A brutal half-giant son of Poseidon. He operates a deadly underground fighting arena, demanding lethal entertainment from his captives and dedicating the skulls of the fallen to his divine father.

The Olympian god of the sea. Though physically distant due to his divine duties and a war in his own underwater domain, he maintains a quiet pride in his children and occasionally offers them cryptic but vital assistance.

The god of wine and the reluctant director of Camp Half-Blood. Despite his usual irritable and dismissive attitude toward the campers, he occasionally reveals a protective streak and shares moments of genuine insight.

An empousa who disguises herself to ambush demigods. She is a manipulative and vicious monster who eagerly gathers intelligence and carries out violent strikes for the Titan army.