Kuroneko to Heishi

Within this battlefield is a cat. Arthur Rain is a mercenary and skilled sniper who is feared as the "Wild Hunt." As he prepares for another day of sniping, a black cat appears before him. A cat that seemed like a stray who had taken up residence on the battlefield, happily playing with an empty bullet cartridge like it was a toy. This is the story of one man and one animal living in a deadly battlefield, trying to make the days a little more lively.

(Source: Kodansha, translated)