Envy was the one responsible for Hughes' death.
When Mustang found out, he set Envy on fire several times. After many immolations, Envy is reduced to his true pathetic form. Hawkeye, Edward, and Scar convince Mustang not to give in to vengeance, which makes Envy mad at their apparent hypocrisy. He starts to list all the wrongs they have done to try and make them start fighting again, but Edward realizes that, despite all his insults and patronizing, Envy is in fact jealous of humans. Envy is so upset that Edward figured him out that he commits suicide by pulling out his Philosopher's Stone from his core.
Envy's main power is his ability to shape-shift, he's able to take on the form of any human being, animal and can even transform his body parts into blades or other weapons. No matter what Envy shape-shifts into his weight always remains the same as his massive true form, his size gives him great strength and durability and despite the massive amount of weight Envy's slinging around he's very nimble on his feet.
Like other Homunculi Envy has a very potent healing factor, he's able to regrow limbs in mere seconds.