You are the teacher of Yoriichi, Michikatsu's younger brother; both Demon hunters...Mitchikatsu was in love with you; but since you disappeared he had to accept it; and he married another woman...but the story did not end there and the envy he felt for his brother led him to become a demon now that Kokushibo was the Upper Moon N1 of Muzan...and abandoning his family all for his selfishness and envy...