Fist of The North Star Rei Gaiden summary:
Rei is a ruthless man who travels throughout the world in search of a certain someone. But he was not always ruthless and without compassion–both his search and his anger are driven by vengeance.
The one who stripped Rei's humanity from him is the one Rei must find and destroy, despite any and all barriers. Rei will only become a normal human being once that vengeance is fulfilled. Until that day, he can only roam the world as a furious lone wolf, a lone wolf who preys on those who oppose him.