Harley is an eight-year-old Greek demigod, a son of Hephaestus, and a camper at Camp Half-Blood.

Harley is first introduced when Leo Valdez visits the forges for the first time. The youngest member of the Hephaestus' cabin, he is seen working along with his siblings to catch the bronze dragon, which is later named Festus (Latin for Happy) by Leo. Harley is affected by the so-called "Curse of Cabin Nine" at the time, which makes everything they build go haywire. At the only time he is seen, he is depressed because of the curse. He has many siblings, including Leo, Nyssa, Christopher, and Jake Mason.