Blame!

Blame! is a manga written and illustrated by NIHEI Tsutomu.

Plot
Killy is a man of few words. He wanders, seemingly endlessly, through a lonely, gargantuan labyrinth of concrete and steel, fighting off cyborgs and other futuristic nightmares, searching only for something called Net Terminal Genes. And he has a very powerful gun, which he uses without hesitation whenever anything resembling danger rears its ugly head. Who is this quiet, violent, determined man and what are these Genes he seeks? The small communities he finds tucked into the crevices of this towering, dystopic ruin hardly give him leads on his treasure, driving him to find larger enclaves of civilization where people can reveal more about the world he lives in and the quarry he seeks.