Envy is the homunculus who has the ability to take on any form he has seen. A sadist who enjoys tormenting humans and sees them as trash.
Envy is a fourth Homunculus created by Father by purging the sin from his body and giving it form with a Philosopher's Stone as his core. He assisted Father in his quest for creating the country sized transmutation circle through the creation of the country Amestris. One step in Father's ultimate goal of creating himself into the perfect being.
For hundreds of years, Envy has gladly played the role of Father his the instigator of wars and took great pleasure in turning humans against one another. While masquerading as an Amestrian soldier during the Ishval uprising, he purposely murdered a small Ishvalan girl and starting the Ishvalan war that sent Eastern Amestris into turmoil, killing thousands.
He's a character who is of an ambiguous gender. Though his appearance is thin and more masculine, he has always been voiced by female voice actresses. Though his hair is intended to be black, the first 2003 anime series colored his hair green. This was a nod to the term "green with envy".
Major Story Arcs
Adventures in the East
Envy was one of the homunculi who assisted their Father in taking over Amestris. His first appearance was after the death of Father Cornello, when he assumed the man's form to call the people of Liore to rebel and fight. After Lust's death, he became the main messenger and instigator of Father's will.
Then, whilst pretending to be Gracia Hughes, he murdered Lt. Col. Maes Hughes in cold blood. Afterwards, during a fight with Edward Elric and Ling Yao, he was swallowed alongside them by Gluttony. Inside Gluttony's stomach, he revealed his unleashed form; a giant green beast. However, he was convinced by Edward Elric to use some of his energy to open the true Gate, which allowed them to escape from Gluttony's stomach.
Later on, Envy releases the "Mad Bomber" Solf J. Kimblee to work for Father, giving him a supplementary Philosophers Stone, made from the souls of the scientists who made the stones during the War on Ishval. However, one of Kimblee's chimera underlings, Zampano, worked alongside Tim Marcoh and May Chang to reduce Envy to his parasitic form.
Envy attempts to hold Yoki hostage to gain his freedom, but to no avail. So, Scar seals his reduced form in a jar and gives it to May, to bring back to her country. However, Envy manages to convince May to turn back to Amestris to "assist her friends". Then, the two encounter several mannequin soldiers, which Envy absorbs, allowing him to reform into his preferred form.
In the end, Envy encounters the Flame Alchemist, Roy Mustang, and the two begin to fight, with Envy revealing that he is the person who murdered Hughes. This led Mustang to mercilessly and relentlessly incinerate Envy around various parts of his bodies, halting only because Riza Hawkeye and Edward Elric intervened, at which time he had once again been reduced in form. He berated the group, until Edward began to speak, and said that Envy was really envious of humans, leading Envy to commit suicide by removing the Philosophers Stone inside him.
Fullmetal Alchemist (2003)
Envy's real face resembles Hoenheim; apparently he was the failed transmutation of Hoenheim, and Dante's son who died prematurely of mercury poisoning. Shortly after creating Envy the Philosophers Stone, Hoenheim abandoned Envy and started fresh with a new family with Trisha. "Needless to say I never did like being replaced" as Envy describes it. Envy also is Dante's most trusted Homunculus for two reasons. One is that Dante took care of Envy while Hohenheim abandoned him, and two Envy was the very first homunculus ever created.
In the series he was last seen opening The Gate to try to get to Hoenheim to kill him, until he transformed into a large Dragon. In the movie it is revealed he made it past The Gate but it is suggested that he is unable to transform back as he never attempts to probably due to the fact that alchemy doesn't exist on the other side of the gate. When the Thule Society captures him they use him as a catalyst to make a permanently opened gate. However this only possible because Hoenheim allowed Envy to kill him so his son could return back to his home. Hoenheim also claims he was a sinner for turning his son into a monster and cheating death by taking life from others after that Envy, Hoenheim aren't seen ever again.