In Percy Jackson and the Olympians: The Lightning Thief, Grover Underwood is Percy's protector. He becomes a central character in multiple plot lines in the Percy Jackson series. While many of the characters in this book are demigods, meaning that they are half human and half god, Grover is a satyr, a mythological creature that is half man and half goat.

Because Percy thinks Grover looks like he was held back a few years, we can infer that Grover looks a bit older than Percy, who is twelve years old. Grover is described as having wispy facial hair and acne and being "scrawny." The most notable feature of Grover's appearance is his legs. When he is not among humans, he is free to look like a satyr, having a man's upper body and the horns and legs of a goat. In the human world, Grover hides his horns with his curly hair or a hat and hides his goat legs with pants and fake human human feet. Because of the way his goat legs work, his human disguise gives him the appearance of being disabled, with "some kind of muscular disease in his legs."