Wild Adapter(2001)

Makoto Kubota wandered through life, not taking things too seriously or looking too deep within himself. His job as the head of the Izumo Group's youth gang kept him pleasantly occupied with yakuza wars, mahjong and assassinations... Until the day he stumbled upon a strange drug called Wild Adapter that produces bizarre side-effects--including death. Forever changed, Kubota becomes entangled with a drifter named Minoru Tokito, and the two form an unlikely companionship that draws them deeper into the mystery of Wild Adapter...

(Source: TOKYOPOP)