The symbol for Hades is his helmet, also know as the helm of darkness. He is also known by his guardian, which was a three-headed dog.
Hades, son of Cronus and Rhea, was the God of the Underworld. It was believed that he ruled the Underworld and this led to the word "Hades" becoming synonymous with death. Hades rarely left the underworld and he had a guardian named Cerebus. This three-headed dog protected the realm by keeping out those who did not belong there. Hades and Cerebus also prevented anyone from leaving the Underworld without their permission.
Hades had a dark personality which was blamed on his long life in darkness. The Underworld was his shrine and he was unpopular with Gods and humans due to his thirst for power. When Hades did venture out of the Underworld, he wore his helmet that he had created to make him invisible. Nobody knew that he ventured outside of this world unless he wanted them to know. Hades did marry, however, and his wife was the daughter of Zeus and the Harvest Goddess Demeter. He stole Persephone from his own sister and brought her to the Underworld where she became his queen. Known as the goddess of vegetation before her marriage, she became queen of the Underworld.