As per the anime, the main plot surrounds finding the gold that was stolen from the Ainu. In which the leader tattooed the location of the gold with his cellmates. Now the plan for a more typical approach is to kill and skin the tattoos, but Asirpa is very much against killing. So just copying the tattoos will be their approach.
Sugimoto is referred to as the “Immortal Sugimoto.” Unsurprising in a kill-or-be-killed world, he always made sure he was on top. In fact, it can be seen when he was younger he was much more demonic. His friend that he made that promise of finding the gold means more to him since Sugimoto & Ume loved each other. Of course Sugimoto was pretty messed up after losing his father and wanting to walk his own path. So him earning even a fraction of the golden is all he really needs. Sugimoto’s past is both tragic and dark.