The title character of the story, Zwei was a perfectly ordinary tourist from Japan, before witnessing an Inferno assassination in America and being kidnapped. He was able to elude his pursuers long enough for them to realize he had a natural survival instinct that was perfect for an assassin. He had his memory erased through a combination of drug and hypnotherapy. The name Zwei is simply a code name to show he is the second experimental assassin created by Scythe Master.

His name means "two" in German.

(Source: Wikipedia)