Kōsetsu is the one responsible for killing Masashige and helps his brother kill Kurohabaki Terumune. He travels the alps with his brothers as an assassin and kills anyone not willing to join the Kurohabaki clan. Recently, he fights Orion and the Ōu army, before being washed away by the strong current of a flood. He survived the flood by clinging to Orion, but his brothers drowned. He starts a fight with Orion before returing to Masamune.
After the war against Masamune is over he comes across Kenshin wanting revenge for the saluki's betrayal of the clan leader which is short lived when help arrives. Kenshin informs him of the leader's death and tries to tell him to give up his life of revenge if he doesn't the Ou Army will find him.
But he refuses and attacks Kenshin until the saluki's subordinates pins him down and he begins to cry. Kenshin tells him the importance of life and that the kai ken tears is his own conscience rebelling against the evil inside him. And steers him in the way of using his life for the good of others making him feel good inside.
Then, he sees a flock of crows flying overhead and ran off to see what the commotion is about. Kosetsu finds Yamabiko and his friends burying Kojiro.
He tries to attack Yamabiko with the clan's special technique but is stopped by Andy and the two Koga ninja dogs. He is told the german shepard the Ou Army doesn't take well those resist. He listens to Yamabiko tell him about how it's the Ohu Army mission to make the world more peaceful for all dogs. The assassin hear how Masamune died by his own hands and doesn't know what to do he attacks Yamabiko but is stopped by him and Orion and quickly grabbed by Gennai and slammed to the ground.
But he is okay but he still swears to Masamune and thought his death would bring close to him along with his brothers. Gennai tells him when someone loses their purpose they learn to seek another, Kosetsu begins to cry learning that Masamune was wrong and Kosetsu walks away.
See more pictures in Kōsetsu (Photo Gallery).