Scar

Scar is a disturbed loner. His right arm does not belong to him, but his brother. His brother was obsessed with finding the Philosopher's Stone and tattooed his whole body. 

During the Ishbal civil war…

an alchemist, Solf J. Kimblee, came and killed his brother, blew off Scar's arm and gave him the 'X' scar on his forehead. Scar's brother than transmuted his own arm onto Scar's shoulder, giving the maimed warrior priest the powers of alchemic deconstruction that he then uses in his quest for vengeance.

Scar seeks his revenge against the state alchemists, all of whom he blames for the destruction of Ishbal. His wrath does not extend, however, to non-state alchemists, which is why he does not seek to kill Alphonse Elric during their frequent confrontations.

In the first anime series; Scar and Lust have some sort of history that is explained all too briefly.

Lust was originally Scar's brothers wife/girlfriend/non-specific lover. When Lust died, Scar's brother attempted a human transmutation to revive her. When this failed, a new homunculus was born (as per the method of created explained in the anime).