Zwei

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.

Spoiler, click to view

Through Claudia, one of the three top executives of Inferno, Zwei is able to learn his true identity as Azuma Reiji.

Spoiler, click to view

When Reiji and Eren finally set out to find her past and end up in Mongolia in a field of Oxytropis flowers, an unidentified assassin kills Reiji with a silenced gun, and he collapses silently to the ground. Ein then appears to commit suicide by eating a leaf of the poisonous flower. It is hinted in the leadup to his death that Reiji saw his end coming, and accepted it, having considered his final task complete.

His name means "two" in German. (Source: Wikipedia)