He is a major character in GDW:O. He's a Kurohabaki Clan's member. He has two brothers Kurokirimaru and Tsumuji. Kosetsu is the one responsible for killing Masashige and helps his brother kill Kurohabaki Terumune. He travels the alps with his brothers as an assassin, he kills anyone not willing to join the Kurohabaki clan. Recently, he fights Orion and the Ohu army,before being washed away by the strong current of a flood. He survived the flood by clinging to Orion. He starts a fight with Orion before returing to Masamune.