A student of Toshihiro Eguchi. According to a recently published Japanese book by Mihashi Kazuo on the life of Toshihiro Eguchi, Kôshi Mitsui was a village mayor who wrote and reviewed waka. He introduced Toshihiro Eguchi’s teachings in a book, which increased their popularity, and he also helped facilitate the actual teachings.
He was also a good friend of fellow student Gorô Miyazaki.