Grover Underwood

Grover is introduced to us as a normal human early on in the book, but he is soon discovered to be a satyr, who is tasked with protecting Percy. He is Percy's best friend and the two of them are joined by Annabeth who is the third companion on their quest to save his mother and inadvertently return the master bolt to Zeus. He is the first to realize that they had stumbled onto the lair of Medusa when he sees his uncle Ferdinand as a stone statue and recalls how he had died.

Annabeth Chase

She is one of the supporting characters in the book. Percy doesn't meet her till he reaches Camp Half-Blood. He immediately likes her and she along with Grover go on the mission to save his mother and inadvertently return the master bolt to Zeus. She is a daughter of Athena, so she is skilled in battle, and Percy earns her respect by beating her in combat during a game of capture the flag.

Percy Jackson

He is the protagonist of the book and series. He is a young teenager as the books go on. He is just a normal kid at first, then his life is changed when he is forced to go retrieve his mother from the underworld, inadvertently carrying Zeus' lightning bolt which is the current fascination of the gods. This adventure leads Percy to change mentally and physically as a character in order to adept to the situations he is facing.