Kaijin was a blacksmith of the Dwarven Kingdom, and the brother of Kaidou. After he was exiled from his own country, he became Rimuru Tempest's subordinate, and helped build the Monster City "Tempest", he acted as a builder, blacksmith, and craftsmen. He also was the former commander of the Dwargon Engineering Corps. (Source: Tensei Shitara Slime Datta Ken Wikia)