Hades

In Mythology: Hades was the ancient Greek god of the underworld. He is also the oldest child of Kronos and Rhea. He is often pictured with the three-headed dog Cerberus. In the later mythological tradition, he is associated with the Helm of Darkness and the bident. He is most famous for abducting the goddess of spring Persephone and making her be his wife.

In the book: In the book is is most known for abducting Percy's mother in the first book while trying to get back his Helm of Darkness which was stolen from him at the same time that Zeus's bolt was taken from him. He is also the father of Nico di Angelo one of the main  characters in the book. 

If you would like to know more about Hades please go to this link:
www.greekmythology.com/Olympians/Hades/hades.html