The oldest surviving Homunculus, Envy claims that he has forgotten what he originally looked like, but in truth chose not to revert to that form out of hatred for his "father" - Hohenheim - whom he resembled. He hates all humankind and follows Dante mostly for the opportunity of killing as many people as he can. This is what drives Envy in all that he does--unlike the other Homunculi serving Dante, Envy does not care about the Philosopher's Stone, nor about becoming human. His lust for revenge on his creator, and inflicting pain and suffering on all humans, was the only thing that mattered. His most prominent murder was that of Mäes Hughes, whom he kills after taking on the form of his wife, Gracia. Envy was also key in imprisoning Greed, against whom he had a long-standing grudge, within the 5th Laboratory.
Envy's original human face, from "Death".
It is revealed in the penultimate episode that Envy was created four hundred years ago by Hohenheim in an attempt to resurrect the son he and Dante had, who had died from mercury poisoning when he was eighteen years old. Envy holds a great hatred toward Hohenheim for never being acknowledged as his son, and for abandoning him and Dante. As a result, he naturally also hates his younger "half-brothers" Ed and Al, as they obtained the fatherly love that Envy himself never received. When learning from Dante that Hohenheim is gone, Envy snaps at being denied his wish to kill him. But the fact that the Elrics possessed the stone and would gain back all that they had lost makes Envy more bent on making them suffer as he had. However, Edward manages to see through Envy's disguises and tricks until the Homunculus reveals his true form (an imitation of Hohenheim's deceased son with Dante) and his connection to Hohenheim. That stunt is all it takes for Envy to successfully kill Ed, though Al manages to revive him with the help of the Philosopher's Stone. Upon learning Hohenheim was still alive on the other side of the Gate, Envy - still obsessed with the thought of killing him - is able to successfully open and make his way through the doors, during which he tranforms into both Edward and the person he was based on. By the time he makes his way through to the other side, Envy becomes a massive amphibious serpentine dragon (a reference to both the Leviathan as the sin of envy personified and the Ouroboros tattoo all Homunculi possess) in the process. His power of transformation is rendered inert as he arrives at the other side (since alchemy does not exist there), trapping him in his serpentine form forever.
During the two year gap between the end of the TV show and the movie, he is captured by the Thule Society, possibly with the help of Hohenheim. With Envy still trapped in his massive serpent form, the Society intends to use Envy as a literal Ourobouros to open a physical Gate and transfer their soldiers to the other side, to "Shamballa." Hohenheim is imprisoned and offered to Envy as a sacrifice; a human chew-toy to keep the "Great Snake" calm as they formulate their plans. Around the time of Fullmetal Alchemist the Movie: Conqueror of Shamballa, Envy escapes, and takes shelter in an abandoned castle only to cross paths with Edward once again. In their ensuing fight, Envy is attacked on all sides by the Thule Society, who recapture him and manage to get a small regiment of soldiers to the other side for a short time. Envy's exact intentions toward his "father" are unclear. In the end, it is Hohenheim who takes his own life using Envy as his means, enabling Ed to return to his own world in a final act of redemption for his sins. Thus, Envy is able to satiate his desire for vengeance on Hohenheim, albeit at the cost of his own life.