| | | 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. |