Greed is a homunculus with the Ultimate Shield. The only homunculus to defect from Father and start a life on his own.
In the manga based series, Greed is introduced to the Elric brothers in very much the same way as in the first series. However after being defeated by Edward, he flees and encounters Fuhrer Bradley who is later revealed to be a homunculus.
Later in the series, Greed is transferred into the body of Ling Yao by Father.Greed does not completely take over Ling Yao's body because the prince doesn't resist him which leads Ling's body to be controlled sometimes by its rightful owner sometimes by Greed.
Shield: Greed has the ability to completely surround his body by a substance that resembles diamond as Edward Elric says while fighting him resembles.This ability earned him the name Ultimate Shield.
Regeneration: Greed can regenerate and come back to life under certain conditions.He cannot regenerate if he is creating his shield at the same time,he can't regenerate if the philosopher stone he possesses as ran out (manga/ brotherhood series) and he can't regenerate in presence of the remains of the dead human he was meant to replace.