Percy is a demigod son of Poseidon returning for his second summer at Camp Half-Blood. He struggles with his status as a forbidden child of the big three gods and manages a complicated connection to his godly heritage. Fiercely loyal, he repeatedly risks his own safety to protect his friends and the camp.

Annabeth is a brilliant demigod daughter of Athena who serves as the tactical mastermind of the group. She lives with one foot in the mortal world and one in the demigod domain, constantly thinking and planning. Her painful past leaves her deeply distrustful of cyclopes.

Tyson is a homeless youth who attends Percy's school and is unusually large for his age. Initially appearing as a timid boy who cries easily, he is revealed early on to be a cyclops and a son of Poseidon. He possesses a natural immunity to fire and a remarkable gift for mechanics.

Luke is a rogue demigod son of Hermes who betrayed Camp Half-Blood the previous summer. Blaming the Olympians for the unfulfilled lives of demigods, he amasses an army of monsters and disillusioned campers to tear down the gods and reshape the world.

Polyphemus is a massive, half-blind cyclops who resides on an island in the Sea of Monsters. He holds a deep grudge against heroes after being blinded in a past encounter. He hoards the magical Golden Fleece to lure satyrs to his island for food.

Kronos is the titan father of the gods whose reforming essence rests in a golden sarcophagus. Though physically absent, his manipulative influence drives the central conflict as he seeks to recruit demigods and overthrow Olympus.

Clarisse is a fierce and aggressive demigod daughter of Ares. She receives the official quest to find the Golden Fleece by the new activities director. Despite her bullying exterior and rash behavior, she is fiercely dedicated to proving herself and achieving glory for her father's cabin.

Grover is a satyr and Percy's best friend who left Camp Half-Blood to search for the lost god Pan. He becomes trapped on a dangerous island in the Sea of Monsters. To stay alive, he utilizes his wits and a bridal-gown disguise to stall his captor.

Circe is a beautiful sorceress who runs an exclusive island spa for young women. Harboring a deep disdain for men, she uses her potent magic to transform male visitors into harmless animals, specifically guinea pigs.

Tantalus is the newly appointed activities director at Camp Half-Blood, replacing Chiron. Cursed by the gods to be forever hungry and thirsty, he is bitter, dismissive of the camp's dangers, and actively hostile toward Percy and his friends.

Hermes is the messenger god and Luke's father. Restricted by godly laws from directly interfering, he provides Percy with magical gifts and subtle guidance. He hopes that Percy might eventually help his rebellious son find a better path.

Chiron is a wise centaur and the former activities director of Camp Half-Blood. Wrongly blamed for the poisoning of the camp's protective tree, he is forced into exile but remains deeply concerned for the safety of his demigod students.

Poseidon is the god of the sea and one of the big three Olympians. While distant due to divine laws, his influence is felt throughout the ocean. He surprises the camp by publicly claiming Tyson as his son.

Thalia is a demigod daughter of Zeus who sacrificed herself years ago to save her friends. Her spirit was transformed into a magical pine tree that protects Camp Half-Blood's borders until it is struck by a mysterious poison.

Ares is the god of war and Clarisse's father. He provides his daughter with a warship manned by undead Civil War soldiers, placing immense pressure on her to succeed and bring glory to their family.

Athena is the goddess of wisdom and strategy. While she does not appear directly, her influence is clear through her daughter Annabeth's sharp intellect and the magical items she provides her children.

Zeus is the king of the gods and one of the big three Olympians. He transformed his dying daughter Thalia into a pine tree to preserve her life force and protect the borders of Camp Half-Blood.

Rainbow is a hippocampus, a half-horse, half-fish sea creature. He forms a quick attachment to Tyson and provides crucial transportation and rescue assistance during their sea journey.