He is known as Scar because of the X-shaped scar on his face. Scar is an Ishvalan and a survivor of the Ishval Civil War against Amestris. Scar lost his right arm in the war but before his brother died he transmuted his arm onto Scar.
This arm is tattooed with rare transmutation symbols and makes Scar a powerful alchemy user. Scar believes that alchemy is a sin against God and the natural world, but it doesn't stop him from hunting down and killing State Alchemists.
In the English dub version of the first Fullmetal Alchemist anime series Scar was voiced by Dameon Clarke.
For the English dub of the Fullmetal Alchemist: Brotherhood anime Dameon Clarke didn't return to reprise the character, so Scar is voiced by J. Michael Tatum.