Hades

Despite being the god of the underworld in Greek Mythology, Hades is not evil like his depictions in modern media, he is actually one of the more well behaved of the major Greek gods.

Hades is a name for the Underworld, but it was named after the god who ruled it.

Pluto is the Latin name of Hades. The Greek underworld is also known as Tartarus. Zeus, Poseidon and Hades were dividing up the world by rolling dice. Zeus got first roll, and go the Sky. Poseidon got the Sea. So, Hades was left with the Underworld. He became bitter and cruel in the Underworld.

I'm pretty sure Pluto was the god of the Underworld, but I think the Disney movie messed a lot of people up. Everything I've read says that Hades was only the underworld, not the guy who worked there.