Phantom: Requiem for the Phantom

A spate of mafia assasinations occurs in America, the work of the mysterious organization named Inferno. "Phantom" is their most strongest and capable assassin, however a lone tourist witnesses Phantom's latest act of murder. He unwittingly exposes the top assassin's true identity, a young girl named Ein. Witnesses are usually killed, but the man is captured and taken to be brainwashed into "Zwei", Inferno's newest assassin, with no memory of who he is. From this point on, Zwei is drawn into a world of conspiracy, deceit and violence.

Synopsis from Anime News Network