Hephaestus

In Mythology: Hephaestus is the God of fire and blacksmiths. He is the son of Zeus and Hera but was thrown off of Olympus because he was ugly. He is most famous for crafting lighting bolts for Zeus.

In the book: He is not featured much in the book however he did craft the item needed for Percy and Annabeth to navigate the Labyrinth. And he also helped Percy  get of Calypso's island at the end of book four.

If you want to learn more go to this link: http://en.wikipedia.org/wiki/Hephaestus