Envy is a homunculus who has the ability of shape-shifting. They use this power to assume the form of any person and even copy their voice. They usually serve as a spy, but their abilities are also extremely useful for assassinations. Envy may not be as smart as some of the other homunculi, but is by no means an idiot. In fact, they can be just as good a puppet master as Lust, but this is overshadowed by their cockiness and bad temper. Their temper can easily be set off by insulting their large and delicate ego, causing them to make mistakes.

In the 2003 anime series:

As the oldest Homunculus, Envy claimed that they had forgotten what they originally looked like. Envy also greatly hates all humans and follows Dante mostly for the opportunity of killing as many people as they could. This is what drove Envy in all that they did and unlike other Homunculi, Envy does not care about the Philosopher's Stone, nor about becoming human.

In the manga and 2009 anime series:

Envy was the fourth Homunculus created by Father. Though often referred to as a "he" Envy is apparently genderless and can take almost any form they choose. Referred to by Lust as the most cruel or sadistic of the Homunculi, Envy shows the most open disdain of humans and relishes in any opportunity to hurt, humiliate, or insult them. They are also quite vain, short tempered, and dislike anyone insulting their appearance.

(Source: Fullmetal Alchemist Wikia)