Grover Underwood is a satyr who is Percy's best friend. Grover is described as having curly brown hair, brown eyes, pimply skin, a wispy beard, and brown goat-legs (which he disguises with fake feet when necessary). His horns grow larger as the series progresses, and he must take increasingly careful measures to hide them while posing as human. He is a vegetarian but also eats tin cans, napkins, furniture, silverware and enchiladas. Chronologically, Grover is in his thirties, but because a satyr's lifespan is twice that of a human, he is still considered a young teenager in his community. Grover is a very sensitive person, concerned particularly for the safety of nature and nature spirits—and like all satyrs, he can sense the emotions of others around him and smell monsters and demigods. As the series progresses, his concern for his friends and pursuit of his goals lead him to take on leadership roles and become more sure of himself. Unlike his demigod friends, Grover is not an orthodox fighter. Instead, he uses a magical flute, which he uses to manipulate nature into his control; at his peak, Grover is able to create physical things, such as a cage, out of nothingness. In The Battle of the Labyrinth, Grover begins a relationship with the wood nymph Juniper.

He is first seen in The Lightning Thief, where he is disguised as a human student at Percy's school, serving as his "protector". Grover later reveals that if he completes a protector mission, he will receive his "searcher's license", and leave to look for the missing god Pan, his life's ambition.[3] When Polyphemus captures him in The Sea of Monsters, he forms a psychic bond with Percy, called an empathy link, allowing them a limited degree of telepathic communication across great distances. He uses this ability to guide Percy to his rescue.[7]He joins Percy's quest to find Artemis in The Titan's Curse. He eventually finds Pan in The Battle of the Labyrinth, and receives the wild god's spirit, though the Council of Elders do not believe his having witnessed Pan's death. At the end of The Last Olympian, he is named a Lord of the Wild and given a seat on the satyrs' ruling council.

In the film adaptations, Grover is portrayed by Brandon T. Jackson.