Wikia - Richard Harrow
Richard Harrow (played by Jack Huston) was a disfigured war veteran, criminal, and friend of the now deceased Jimmy Darmody. He acted as Darmody's bodyguard and bootlegging partner. Richard Harrow is a seemingly calm but deeply troubled and ultimately tragic character. He is not just surrounded by death but finds himself useful chiefly as an instrument of death.
Wikipedia - Richard Harrow
Richard Harrow was a sniper in the US army during World War I. He was badly disfigured during the war, losing his eye, upper jaw and most of the cheekbone on the left side of his face. He has also suffered some throat damage which causes him to speak in a low, muffled voice. Because of his appearance, Harrow wears a tin mask in public that was molded to match the left side of his face.
After the war he returned to Wisconsin where his twin sister, Emma, cared for him while his wounds healed. Once he was able to care for himself he left to live in Chicago.
IMDb - Richard Harrow
Richard meets Jimmy Darmody at an Army hospital in Chicago and, after sparking an instant kinship, invites him to return to Atlantic City and join his bootlegging operation. Harrow began work as a hitman and hired muscle for Jimmy's bootlegging enterprises, immediately demonstrating his loyalty to Jimmy and becoming protective of him and his family. Richard's sharp-shooting skills, not surprisingly, come in handy as Jimmy exerts his authority around town.
Despite his exterior, Richard is a romantic who keeps a scrapbook that contains images of domestic life and love clipped from various periodicals, lamenting a life he worries he'll never have.
HBO - Richard Harrow
A survivor of the Great War and an expert marksman, Richard Harrow wears a mask to cover the injury that took half his face. Fellow veteran Jimmy Darmody bonded with the sensitive and self-conscious Harrow when they met in an army hospital in Chicago. Harrow returned to Atlantic City with Jimmy, and after the death of Jimmy and his wife, has been keeping a watchful eye over their son, Tommy.