| | | | Camp Half-Blood - Demigod's training camp. |
| | Clarisse | La Rue | Percy's old enemy from Camp Half-Blood. Ares's daughter. |
| | Chiron | | Half man, half horse. Activities Director at Camp Half-Blood. |
| | Kronos | | Zeus's father. The scariest titan. |
| | Annabeth | Chase | Percy's best friend. Athena's daughter. |
| | Luke | | Annabeth's friend. |
| | Thalia | | Zeus's daughter. Demigod. |
| | Grover | Underwood | Half man, half goat. |
| | Tyson | | Cyclops. Percy's brother. |
| | Poseidon | | Percy's father. God of the sea. |
| | Ares | | Clarisse's father. The God of War. |
| | Prissy | Jackson | Poseidon's son. Protagonist. aka: Percy. |
| | Mrs. | White | Percy's teacher. |
| | | | Riptide - Percy's ballpen-looking weapon. aka: Anaklusmos. |
| | Phobos | | Clarisse's immortal brother. |
| | Deimos | | Clarisse's immortal brother. |
| | Sherman | | Clarisse's mortal brother. |
| | Mark | | Clarisse's mortal brother. |
| | Mr. | Bell | Percy's woodwork teacher. |
| | Bartholdi | | Athena's son. |
| | Athena | | God of Wisdom. Annabeth's mother. |
| | Jessie | | Girl in the zoo. |
| | Connor | Stoll | Camp store thief. Hermes's son. |
| | Travis | Stoll | Camp store thief. Hermes's son. |
| | Michael | Yew | Guy in Apollo Cabin. |
| | Lord Zeus | | God of Thunder. Dionysus's father. |
| | Apollo | | God of Music. |
| | Dionysus | | Camp Half-Blood director. God of Wine. |
| | Hephaestus | | Zeus's son. Charlie's father. |
| | Hermes | | Apollo Cabin member. God of the
Roadway. |
| | Aphrodite | | God of Love. Member of Athena's team. |
| | Demeter | | God of Grain. Member of Athena's team. |
| | Charlie | Beckendorf | Lead councilor of Hephaestus Cabin. |
| | Silena | Beaurard | Head councilor for Aphrodite. |
| | Myrmekes | | Monster. Means "ants" in ancient Greek. |
| | Artemis | | God of Archery. |
| | Hera | | Zeus's wife. |
| | Hades | | God of the Underworld. |
| | Persephone | | Hades's wife. |
| | Odyseuss | | Greek hero. Inventor of ear wax. |
| | Arachne | | A great weaver. |
| | George | | Hermes's snake. Martha's twin snake. |
| | Martha | | Hermes's snake. George's twin snake. |
| | | | The Sirens - Sea monster. |
| | Circe | | God of Magic. Runs a fashionable spa and resort. |
| | Polyphemus | | Elder cyclops. Poseidon's giant son. |
| | Mrs. | O'Leary | Percy's pet hellhound. |
| | Dr. | Boring | Percy's english teacher. |
| | Bianca | | Artemis's hunter. Deceased. |
| | Nico | | Bianca's little brother. Hades's son. |
| | Sisphus | | King of Ephrya. The man who cheated death. |
| | Thantos | | The reaper of souls. |
| | Melinoe | | Demigod. |
| | Ethan | Nakamura | Nemesis's son. |
| | Paul | | Thalia's stepfather. |
| | | | Keres - Battlefield spirits. |
| | Lapetus | | Kronos's brother. Atlas's father. aka: Bob the Titan. |
| | Atlas | | Lapetus's son. |
| | | | Aegis - Thalia's shield of weapon. |
| | | | Backbitter - Deadly sword owned by Luke. |
| | | | Yankees BB Cap - Annabeth's weapon. Gift from her mother. |
| | | | Tyson's Shield - A wristwatch made by Percy's brother Tyson. |
