Percy is the narrator of the story, a teenage boy who is 14 years old. He is a demi-god and the son of Poseidon(the god of water). Since he is Poseidon's child, he can swim without worrying about shortage of breath, and can communicate with animals, especially sea creatures. Percy owns a ball-point pen that was crafted by Zoe Nightshade, one of Artemis' hunters. When the cap of his pen is taken off, it becomes into his sword, Riptide. In case of emergency, Riptide is programmed to never leave Percy, so if he loses it, it appears back in his pocket.
Annabeth is a 14 year old demi-god who is the daughter of Athena. She has a passion for architecture, and wishes someday to become an architect. She owns a Yankees hat, but it is no ordinary hat. It's Annabeth's Cap of Invisibility, allowing her to get anywhere without being seen. In the Titan's Curse, a manticore, a monster, took Annabeth away from the rest of the group, and her friends venture to go save her.
Grover is a satyr, which is half human and half goat. Grover in this case has goat-like features such as horns, hooves, and legs just like a goat. In order for Grover to be in the presence of other mortals outside of Camp Half-Blood, he must wear a hat to cover his horns, and fake plastic legs/feet to hide his goat-like legs. Along with Annabeth, Grover and Percy are best friends. Although Grover appears to be 14 years old, he is actually 28 because satyrs have a slower growth rate than humans. Someday, Grover hopes to find the god of satyrs and nature, Pan.
Thalia is a 15 year old demi-god, and daughter of Zeus. Her personality type is described as "independent and many times sarcastic". Thalia's father is the "god of the sky" but yet, embarrassing enough, she admits to Percy that she has a fear of heights. The "Big Three", also known as Zeus, Poseidon, and Hades, swore an oath to never have demi-god children. When Thalia was born, she was a big disgrace. Hades, the god of death/the underworld, sent monsters to Camp Half-Blood. Before Thalia could reach through the protective border around camp, the monsters attacked. Thalia was majorly injured, but Zeus took pity. He preserved her by turning her into a pine tree, so she wouldn't die and have to go to the Underworld, where Hades controlled. Later at the end of the second book, The Sea of Monsters, Grover and Percy awaken her from her tree by using The Golden Fleece.