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. Leo comments that all of the kids in this cabin look different, saying, "You'd never think, 'Hey! It's the Hephaestus bunch!'"
Harley is first seen explaining the rules of the Three-Legged Death Race, which everyone is annoyed about. However, nobody dared to protest, because the race was Harley's idea and they did not want to disappoint him. The following day he started the race.
When Austin and Kayla went missing during the race, he became very upset. He cheered up when Apollo gave him advice for the Home-Beacon he was trying to build in order to help Leo. In return, he gave the former god a combat ukulele, which he had made himself.
He helped defending the camp from Nero. Harley was very mad at Leo when he returned, and he even punched him. But after that, he gave Leo a hug, cried, and ran away. It's revealed that what eventually allowed Leo to returns was Harley's beacon after he had tuned it with Apollo's help. Before catching the beacon's signal, Leo, Festus and Calypso were lost in the Sea of Monsters.
Harley is seen challenging new campers to arm wrestling matches.
In The Lost Hero, he was described as beat-up and depressed, probably because of carrying the weight Curse of Cabin Nine. Harley is also very strong, as he works on the forges. Leo describes him by saying that "even little Harley looks like he could probably go six rounds with Chuck Norris without breaking a sweat". Harley's hands are described as being large and calloused.
In The Hidden Oracle, Harley is said to be "scarily buff for for an eight-year-old" and to have an infectious smile that can lift the mood of people at Camp Half-Blood. Apollo described him as a cute little pipsqueak. He is also said to have a mad gleam in his eyes that resembles Hephaestus when he invents a new automaton.
As a child of Hephaestus, he has the following abilities:
- He is an expert builder.
- He is an expert mechanic.
- Technokinesis: As a son of Hephaestus, he can control machines.
- He can sense machines, mechanics, or metal.
- He can identify machinery type and use by touch.
- He can sense faults in metal ore.
- He can find and sense traps in the ground.
- It is hinted that Harley is named after "Harley Davidson," the popular motorcycle company.
- He is technically the youngest demigod to appear at a camp in either series.