The Lightning Thief
We never meet Hephaestus officially, but we see his handiwork when Percy and Annabeth get trapped in the Tunnel of Love ride – a trap meant for Aphrodite and Ares. Hephaestus is always trying to publically embarrass them in front of other Olympians. His dad, Zeus, threw him over a cliff when he was a baby, because Zeus thought he was ugly. Hephaestus is an incredibly talented blacksmith. His demi-god kids in cabin #9 are skilled at making things like the coolest fireworks you've ever seen in your life.