Other protagonists in the book include:
Grover Underwood: Percy's best friend, a satyr who disguises himself as a student at Percy's school.
Annabeth Chase: A brave and resourceful daughter of Athena, the Greek goddess of wisdom.
Tyson: A Cyclops and half-brother to Percy.
The main antagonist in the novel is Luke Castellan, son of Hermes. Luke was once a respected camper at Camp Half-Blood, but he turned evil after being manipulated by the titan Kronos.
Other antagonists in the book include:
Ares, the god of war, who is manipulating Luke and using him to achieve his own goals.
Medusa, a terrible monster who turns anyone who looks directly at her into stone.
The Minotaur, a fierce half-man, half-bull creature that lives in the labyrinth below the camp.