Voiced by: Ryotaro Okiayu (Japanese), Stephen Holgate (English)
A follower of Yagyu Jubei, he carries the task of searching for a worthy successor to his master. After 300 years, he met Jiyu in the modern day Japan. He tries hard to persuade her to wear the Eyepatch which she quickly refuses. He then takes up residence with the Nanohanas and builds his own Koinosuke's hut (doghouse) although Jiyu said he could sleep in the house he says that he doesn't want her to spoil him. He sometimes transforms into a 300-year old old man for not fulfilling his task. Despite his appearance, Koinosuke states that he has no fighting skills.
His life centers around the mission he was given by his former master and he struggles to find a new life after he Jiyu tells him. In the end, he is reunited and served his master in the afterlife.
Koinosuke appears briefly in the second series to support Jiyu and his child.