Genjimaru

Genjimaru is an old, but extremely powerful mononofu serving under Kūya. He is also Hien and Sakuya's grandfather. His main weapon of choice is a long sword. He was instrumental in helping the other nations defeating the Avu-Kamuu, by revealing their weakness. He would later be killed by Dii as "payment" in canceling Kūya's contract.

Source: Wikipedia