Envy

History
Envy was the fourth Homunculus created by Father. 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 even insult them. It is also quite vain and short-tempered and dislikes anyone insulting its appearance. Beneath this cruel exterior, however, Envy is secretly jealous of humans' inner strength and the mutually beneficial friendships that they share, as most of its kind show little real concern or emotional support for each other.

Powers/Abilities
Envy has the ability to shape-shift into anything or anyone, should it be real people or human aspects that it made up. It can transform to anything that doesn't exceed the size and weight of his true form, which is very large, not creating much of an issue. This power comes in handy to go anywhere unnoticed or to travel fast, allowing it to shape-shift into an average citizen to blend into a crowd for stealth, or a fast moving animal, such as a horse or dog. Additionally, Envy can transform parts of its body, often using this ability in battle to transform its limbs into weapons.

Envy can also like all of the other Homunculi can regenerate the wounds from battle almost instantly but if Envy keeps on getting blasted and blasted the regeneration process begins to slow down and could lead the Homunculi to death (Example: Mustang kept on blasting Lust and her regenerating power couldn't help anymore and she perished). In addition to the standard regeneration, longevity, shape-shifting and apparent immortality of the Homunculi. It can assume a much larger, monstrous shape with a long tail, an elongated face, a long black mane, four arms and four legs, and a multitude of human bodies and faces protruding from its flanks and shoulders. These are physical representations of the dozens of souls of Xerxes within its Philosopher's Stone, all saying independent things which Envy uses to distract and unnerve its opponents.

However, Envy can only change its size and shape, but not its mass, therefore it weighs much more than a normal sized being and often creates cracks or deep impressions in the ground that are easily recognizable. If it suffers too much damage, Envy is reduced to a tiny parasitic lizard-like form, which resembles a fetal version of its full sized body. In this state, it can merge with and control other organic beings and use them as its host body. In the case of lesser homunculi (example Father's artificial soldiers), it can absorb them and their Philosopher's Stones, completely reforming its original body. Envy's fighting skills are questionable, however. While it does have great physical strength and some fighting ability, it was easily beaten down by Lan Fan and Lin Yao when it fought them. It seems to be unable to deal with skilled fighters in general, and is usually not used for combat purposes. In its monstrous form, it can inflict and resist a great deal of physical damage, but is still extremely weak to certain attacks, such as Mustang's flame alchemy. It uses shape-shifting well in combat, but even then, its skill is lacking against stronger opponents (although this is possibly more due to its sadism to physically or emotionally torment its opponents rather than directly fighting them as well as its anger and frustration clouding its judgment). Envy operates mainly as an assassin and manipulator, using its shape-shifting abilities to impersonate and manipulate unsuspecting humans. Disguised as a state military officer, it shot an Ishvalan child that started the uprising, which eventually lead to the Ishval massacre. While masquerading as Gracia Hughes, it also shot and killed Lt. Col. Maes Hughes when he discovered the secret of the founding of Amestris, an act which earned itself the unknowing hatred of Roy Mustang. After Lust's death, it replaced her as Father's messenger.

(From the Fullmetal Alchemist Wiki)