A description of most characters featured in various mythology series by Rick Riordan.

Perseus Jackson, commonly known as Percy Jackson, is a demigod, son of the mortal Sally Jackson and the Greek god of the sea, Poseidon. Percy lives on the Upper East Side of Manhattan but found his life uprooted upon discovering his true paternity. He has black hair and sea-green eyes. He has inherited special abilities from Poseidon, which include the ability to control water, boats, and ships; create small hurricanes; breathe and see clearly underwater, and talk to horse-like creatures and most aquatic animals. He is also a gifted swordsman using his shape-shifting sword pen Anaklusmos (ancient Greek for "Riptide") for battle. The pen was created by Zoë Nightshade and, when uncapped, changes into a celestial bronze sword. If he loses it, it will always appear back in his pocket.

Percy struggles significantly in his mortal life. He, like most demigods, was diagnosed with ADHD and dyslexia. This made him the subject of bullying from his peers. His mother was once married to Gabe Ugliano (Smelly Gabe), an abusive, gambling-addicted alcoholic whose repugnant mortal odor masked Percy's demigod scent, hiding him from monsters. At the end of The Lightning Thief, Sally turns Gabe into stone using Medusa's head. Later, she marries Paul Blofis, whom she genuinely loves. Sally is later revealed in The Hidden Oracle as to have been pregnant, later giving birth to Estelle. Percy begins dating Annabeth at the end of The Last Olympian.

Percy is the first-person narrator in the Percy Jackson & the Olympians series and The Senior Year Adventures series.

In the films, Percy Jackson is portrayed by Logan Lerman. In the musical, he is portrayed by Chris McCarrell. Walker Scobell portrays the role in the TV series, with Azriel Dalman playing a younger Percy in flashbacks.

Grover Underwood is a satyr and Percy's best friend. He has curly reddish-brown hair and fur, acne, and a wispy goatee. His horns grow larger as the series progresses, and he must take increasingly careful measures to hide them and his goat legs while posing as a human. When we first meet him, he is introduced as walking with crutches, which we eventually learn are to hide his goat-like trot. Also, he has a taste for enchiladas. Percy says that Grover, ignoring his crutches, seems to jump around when enchiladas are served at school. In The Lightning Thief, Chiron states that Grover is small even for his age: He is twenty-eight then, but because satyrs mature half the speed of humans, he is considered a teenager. Grover is quite sensitive and attached to nature. Like all satyrs, he can sense emotions and sense monsters and demigods. As the series progresses, his concern for his friends and the pursuit of his goals leads him to take on leadership roles and become more confident. Unlike his demigod friends, Grover is not an orthodox fighter. Instead, he uses reed pipes or a cudgel. In The Battle of the Labyrinth, Grover is bestowed a part of Pan's essence by the Lord of the Wild himself Pan. Grover also begins a relationship with the dryad Juniper.

In The Lightning Thief, he gets a "searcher's license" after delivering Percy safely, allowing him to search for the lost god Pan. When Polyphemus captures him in The Sea of Monsters, he activates an empathy link, a psychic bond with Percy created a year before that allows telepathic communication across great distances. He uses this to guide Percy to his rescue. At the end of The Last Olympian, he is named a Lord of the Wild and given a seat on the satyrs' ruling council, the Council of Cloven Elders.

In The Heroes of Olympus series, Grover mainly appears as one of Camp Half-Blood's satyr allies reporting on Gaea's rising and participating in negotiations with the Romans to get Reyna to help transport the Athena Parthenos statue across the world.