幻影

Known as the strongest assassin, "Phantom" encounters a young boy named Reiji Azuma, who is traveling alone in the United States. The boy soon discovers that this Phantom, who has been carrying out a series of assassinations targeting mafia executives in the criminal underworld, is actually a girl named Ein, who is around his own age. Later, the boy is kidnapped by a mysterious organization called "Inferno," brainwashed, and loses his memories. He then undergoes training under Ein, learning various assassination techniques, and becomes a killing machine named "Zwei," working for "Inferno" just like "Phantom"... The story also depicts the emotional development between the boy and Phantom.