Hephaestus (Vulcan)'s Clique: Pyros
You know how you were told when you were little to never play with matches? Well, Hephaestus and his flame-loving friends just never got the message. They like nothing better than to watch something burn. Setting trash cans on fire, throwing M-80s into toilets, setting off fireworks in the auditorium... You name it: if it has to do with fire or explosions, these kids have given it a shot.
Pele
The big mama of Hawaiian goddesses, the mistress of volcanoes is said to live in Halemaumaui crater, which is still fiery and smoking to this day. Rumor has it that every eruption on Hawaii's Big Island is an expression of Pele's longing for her lost love, Lohiau. This links her to Hephaestus, who's the Greek god of both fire and volcanoes.
Ika-Zuchi-no-Kami
A Japanese Shinto demon who causes the rumblings of earthquakes and volcanoes. Hephaestus may not be the Greek god of earthquakes (that honor belongs to Poseidon), but he is the god of volcanoes.
Loki
The Norse trickster god, Loki, with his flaming red hair, is a fire god. That's where his similarity to Hephaestus ends, though.