Greed

is one of the Homunculi, artificially created humans as a result of human transmutation.

In the anime
The second oldest of the Homunculi, Greed was created by Dante. Greed can change the density of his body to that of carbon, thus becoming the "Ultimate Shield," to complement the "Ultimate Eye" of King Bradley. Greed was killed by Edward Elric after Edward Elric figured out how to transmutate Greed's body structure, allowing him to pierce Greed's heart, killing him due to all the Red Stones perviously leached out of him.

In the OVA "Reflections", a segment possibly reveals Greed's past and the reason he broke away from the rest of the Homunculi. A voice-over by Envy stated that Greed was Dante's former lover and the current him still loves her. Envy goes on to state Greed was merely an experiment. He denies being some love-struck fool and storms off in a huff.

In the same segment Greed states that Envy always did like to torment him, so the story could actually have been a lie. But seeing how even though he was dying, Greed didn't inform Edward Elric of Dante's involvment with the Homunculi; and the fact he backed down from harming Edward when he noticed Dante was watching, Envy's story could actually be true.

In the manga
Shortly after that "father" melts Greed. Ling is then transformed into the new Greed after being spat out of Gluttony's fake Gate Of Truth.