Percy is a 15-year-old demigod and the son of Poseidon. Approaching his sixteenth birthday, he faces the pressure of a long-standing prophecy that dictates he will make a choice to either save or destroy Mount Olympus. He frequently moves between the magical world of Camp Half-Blood and his mortal life in New York. Armed with his magical sword, Riptide, he leads the defense of Manhattan against the encroaching Titan forces.

Annabeth is a demigod and the daughter of Athena. She aspires to be an architect and brings tactical intelligence to the camp's defense efforts, utilizing magical technology like Daedalus's video shield. She struggles with her shared history with Luke, feeling responsible for his current state and believing she could have changed his path.

Nico is the 12-year-old demigod son of Hades. Frozen in time during World War II, he is an outsider at Camp Half-Blood who spends more time communicating with the dead than interacting with the living. He searches for information about his mother's past and devises a dangerous plan to prepare Percy for his fight against the Titans.

Grover is a satyr and Percy's closest friend. Deeply connected to nature, he feels the distress of the natural world and utilizes his magical reed pipes to open passages and command plant life. He assists the demigods in organizing the defense of Manhattan.

Rachel is a mortal teenager from a wealthy family who possesses the rare ability to see through the magical veil known as the Mist. She experiences prophetic visions and absorbs messages meant for Percy. Her involvement in the magical conflict creates friction with her father, who wants her to attend a restrictive private school.

Kronos is the Titan lord of time who seeks to overthrow the Olympians and destroy Mount Olympus. He has possessed the body of a young demigod to anchor himself in the physical world. He leads an army of monsters, lesser gods, and disillusioned demigods in an assault on Manhattan.

Luke is a demigod and the son of Hermes. Feeling abandoned by his father and traumatized by his mother's instability, he turned against Camp Half-Blood and allied himself with the Titans. He bathed in the River Styx to become invulnerable, allowing Kronos to use him as a vessel for the war against the Olympians.

Poseidon is the god of the sea. He ages physically according to the stress of his domain and currently fights a massive war against Oceanus under the sea. He commands aquatic armies but remains restricted from directly assisting the defense of Mount Olympus due to his own battles.

Chiron is a centaur who serves as the assistant director of Camp Half-Blood. He acts as a trainer and advisor to the demigods, coordinating their efforts and delivering strategic insights regarding the Titan invasion.

Ethan is a demigod and the son of Nemesis. He fights on the side of the Titans because he resents the Olympians for refusing to grant his mother and other minor gods proper recognition or thrones on Mount Olympus.

May is a mortal woman with the gift of sight and the mother of Luke. She attempted to become the Oracle of Delphi years ago but suffered a terrible curse from Hades. She currently lives in a state of delusion, baking cookies and waiting for her son to return home.

Clarisse is a fierce demigod and the daughter of Ares. She initially refuses to participate in the defense of Manhattan due to a dispute over a stolen chariot. She acts as a highly skilled fighter and commands the respect of her cabin members.

Silena is the daughter of Aphrodite and a senior counselor at Camp Half-Blood. She suffers heavily from the casualties of the war, particularly the loss of her boyfriend. She attempts to persuade the Ares cabin to join the fight in Manhattan.

Thalia is an immortal daughter of Zeus who physically remains sixteen years old. She leads a pack of female warriors called the Hunters of Artemis. Before joining the Hunters, she lived on the streets with other runaway demigods.

Charles is an 18-year-old demigod and the son of Hephaestus. He serves as a senior counselor at Camp Half-Blood and specializes in explosives and magical technology. He orchestrates the infiltration of the Princess Andromeda cruise ship to strike an early blow against the Titans.

Hades is the god of the Underworld. He holds a grudge against the other Olympians and wishes for his own son to fulfill the Great Prophecy instead of Percy. He controls armies of the dead and guards secrets concerning his family's past.

Hermes is the messenger god and the father of several demigods. He understands the mechanics of prophecy and fate, which prevents him from interfering directly with his children's choices, a restriction that causes deep emotional rifts within his family.

Hestia is the goddess of the hearth. She appears as an unassuming young girl tending to fires. She provides Percy with food, safe passage, and crucial visions of the past, representing stability and yielding power rather than overt force.

Tyson is a Cyclops and Percy's half-brother. He lives in Poseidon's realm and participates in the underwater defense of the sea kingdom while Percy returns to the surface.

Sally is Percy's mortal mother. She lives in New York with her husband Paul. She supports Percy's dangerous life as a demigod, asking only for a blue signal to know he survives the impending war.

Paul is Percy's mortal stepfather. He lives a normal life in New York but is gradually drawn into the reality of the mythological conflict surrounding his stepson.

Zeus is the king of the gods and the lord of the sky. He focuses his attention on battling the monster Typhon in the Midwest rather than defending Manhattan directly. He maintains strict rules regarding interactions between gods and mortals.