Jason Grace wakes up on a bus, not remembering anything. He, Leo, and Piper come across storm spirits, where Annabeth finds them and takes them to Camp Halfblood. There, Jason recognizes some Greek gods, but by their Roman aliases. He has no trouble understanding that he is a demigod. His father is Zeus, or Jupiter: the god of the sky and thunder and the ruler of the gods. The camp counselor, Chiron, notices a tattoo on his arm that read 'SPQR' and had twelve stripes. He has a sister, Thalia, who is a hunter of the goddess Artemis. Later, Jason discovers that he came from a Roman demigod camp on the West coast.
Piper believes she is Jason's boyfriend, since Hera had altered her memory. She is a daughter of Aphrodite, though she tries to downplay her beauty. She had choppily cut her own hair and she doesn't wear makeup. She has the ability to charmspeak: she can magically persuade people to do things. Her father is a movie star and is very busy, so she does crazy things to get his attention. She unintentionally charmspeaked a car salesman into giving her a car for free, which landed her in Wilderness Camp, where she meets Jason and Leo. Although her relationship with Jason had been a trick of the Mist in the beginning, they eventually developed a real relationship.
Leo is best friends with Jason and Piper. Growing up, he ran away many times. Tia Callida, his babysitter, frequently tried to kill him. Gaea had made him kill his own mother with fire when he was eight. He has the ability to control and start fires, and he is not hurt by them. He is a son of Hephaestus and is very talented with tools. He recovered the metal dragon the Hephaestus cabin had built years ago, and named him Festus. He masks his grief and fear by joking around all of the time.
Annabeth is a camper at Camp Half Blood. She finds Jason, Leo, and Piper, believing they are key to finding her boyfriend, Percy Jackson, who has been missing for weeks. She is a daughter of Athena, and a very talented architect- she was given the assignment to recreate Olympus after the Titan war. She shows Piper around and explains to her that her memories of Leo and Jason were a trick. She doesn't join Leo and Piper on their first quest, but she goes with them when they visit the Roman camp.
Hera is the Greek goddess of marriage and family. Her Roman name is Juno. She disguises herself as Tia Callida, an old lady who taunts Leo and tries to kill him as a child. She is trapped, and needs Jason and his friends to rescue her and stop Gaea from coming to power.
Tristan Mclean is Piper's father, a very famous movie star who rarely spends time with Piper. He is kidnapped to lure Piper and her friends to giants who planned to kill them. He is rescued, and his memories of the incident are erased. Although he loves Piper a lot, she feels ignored by him, since he is so busy and doesn't spend much time with her.
Percy is Annabeth's missing boyfriend. He was the hero of the Titan war, and has done many other heroic things for the camp. He had gone missing around the same time that Jason appeared. Jason, Piper, Leo, and Annabeth deduce that he is at the Roman camp.
Coach Hedge was a leader at Wilderness School, where the bus Jason woke up on was heading. He is later revealed to be a satyr, a creature that is half man and half goat. He has an overconfident personality, which resulted in him being captured by the giant Enceladus after the group is attacked by venti.
Gaea is Mother Earth. She first appears as a mysterious, sleepy figure who convinces Leo to set the room his mother was locked in on fire when he was very young. She intended to use her giants to destroy Olympus and tear modern civilization apart.