Percy Jackson: The son of Poseidon. He’s the sassy, loyal, and incredibly powerful leader who repeatedly saves the world from Titans and Giants. Annabeth Chase: The daughter of Athena. She is the brilliant architectural mastermind, master strategist, and Percy’s partner in just about everything. Grover Underwood: Percy's best friend and protector. Technically a satyr (not a human demigod), but he is a core part of the camp who becomes a Lord of the Wild. Luke Castellan: The son of Hermes. He starts as the cool older brother figure of the camp but becomes the tragic antagonist after betraying the gods for Kronos. Clarisse La Rue: The daughter of Ares. Camp Half-Blood's resident bully-with-a-heart-of-gold who proves her fierce loyalty and bravery in battle. Nico di Angelo: The son of Hades. A powerful, brooding demigod who controls the dead and shifts from an outcast to a vital ally.

Chiron is the immortal centaur who serves as the wise, patient, and deeply caring Activities Director of Camp Half-Blood. For millennia, he has trained the world's greatest heroes—including Hercules, Achilles, and Percy Jackson. To blend in with the mortal world and navigate the camp, he uses a magical wheelchair to hide his horse half, often posing as a mundane Latin teacher named Mr. Brunner.

Zeus: King of the Gods and ruler of the sky. He wields the master bolt and is known for being powerful, proud, and quick to anger. Poseidon: God of the sea, earthquakes, and horses. Percy’s father, who is generally more easygoing than Zeus but terrifying when provoked. Hades: God of the Underworld and wealth. (Though technically not having a throne on Olympus, he is one of the "Big Three" brothers alongside Zeus and Poseidon). Hera: Queen of the Gods and goddess of marriage and family. She is fiercely protective of her own image and famously dislikes demigod children of her husband's affairs. Athena: Goddess of wisdom, warfare strategy, and crafts. Annabeth's mother, who values intellect, planning, and weaving over brute force. Ares: God of war and violence. He is hot-headed, loves physical conflict, and rides around on a motorcycle. Apollo: God of the sun, music, poetry, archery, and prophecy. He is flashy, loves creating terrible haikus, and is the twin brother of Artemis. Artemis: Goddess of the hunt, the moon, and young girls. She leads a band of immortal Maidens (the Hunters of Artemis) and generally avoids men. Hermes: Messenger of the gods, guide of travelers, and god of thieves and merchants. He has wings on his shoes and is the father of Luke Castellan. Aphrodite: Goddess of love, beauty, and desire. She can change her appearance based on what the viewer finds attractive and loves romantic drama. Hephaestus: God of fire, metalworking, and blacksmiths. He is rejected by the other gods for his appearance but is a brilliant inventor who creates incredible automatons. Dionysus (Mr. D): God of wine, madness, and theater. He serves as the camp director of Camp Half-Blood as a punishment from Zeus, and he purposefully gets everyone's names wrong. Demeter: Goddess of agriculture and the harvest. She is obsessed with cereal and constantly tells people to eat their grains.

Kronos: The Lord of Time and King of the Titans. He is the overarching "Big Bad" who seeks to overthrow the gods, manipulating events from Tartarus before possessing Luke Castellan's body to lead the Titan army. Luke Castellan: The former counselor of the Hermes cabin. Driven by resentment toward the neglectful gods, he betrays Camp Half-Blood and serves as Kronos's primary general and physical vessel. Atlas: The Titan of Endurance. Tasked with holding up the sky, he escapes his burden temporarily to orchestrate the abduction of Annabeth and Artemis, acting as a brutal, powerhouse villain. The Monsters: An endless array of ancient mythological threats sent to destroy the heroes, including Medusa (who turns people to stone), the Minotaur (the horn-wielding beast), and Alecto (Percy’s fury-in-disguise algebra teacher).

Chaos: The original void and the first entity to ever exist. Chaos is the source from which all other primordial beings and the universe itself sprang. Gaia: The embodiment of the Earth (Mother Earth). She acts as the main antagonist in the Heroes of Olympus sequel series, waking up to reclaim the world from the gods and demigods. Tartarus: The literal embodiment of the bottomless, toxic abyss of the Underworld. When Percy and Annabeth fall into his domain, he takes on a terrifying physical form that is nearly impossible for mortals or even Titans to fight. Ouranos: The personification of the sky. He was the tyrannical first ruler of the universe before his son, the Titan Kronos, overthrew and chopped him to pieces. Nyx: The goddess of night. She resides deep within Tartarus and is so ancient and powerful that even Zeus is terrified of angering her. Akhlys: The personification of misery and the poison of the world. She is a frail, horrific creature whom Percy famously almost kills by controlling her own tears and poison against her. Aether: The personification of light and the upper atmosphere. The literal son of Erebos (Darkness) and Nyx (Night), he represents the pure, shining air that the gods breathe on Olympus, acting as the cosmic opposite to his father.