Koroshiya Ichi

Ichi The Killer is a completed seinen manga by Yamamoto Hideo.

Plot
In Shinjuku Kabuki-Cho, the biggest sin town in Japan, there are two crazy guys. Ichi is a coward and a wimp, but once he breaks into tears, he turns into a human weapon. With great martial arts skill and a blade hidden in his boots, he butchers a bunch of enemies into mincemeat in the blink of an eye. His arch enemy is Kakihara, a yakuza boss who's tired of everything, including himself. Only when he pierces, cuts, and burns his own and other people's bodies does he feel that he is alive.