Who is Grover Underwood's enemies?

Grover Underwood, a character from the "Percy Jackson & The Olympians" series by Rick Riordan, primarily faces enemies related to the mythology and supernatural world, including monsters like the Cyclopes and the sorceress Medusa. Additionally, he contends with the overarching threat of the Titan Cronus and his followers. As a satyr, Grover is particularly concerned with protecting nature and often finds himself at odds with those who threaten the environment, such as the environmental villain, Luke Castellan.