Koshinori's wife died after she gave birth to Kyuubei. Some of the relatives urged him to remarry to have a male heir, but he refused, out of his deep love for his late wife. He feared that if he remarried and had a son, Kyuubei would lose her position in the family. He decided to hide Kyuubei's gender from everybody and trained her to be the rightful heir to the family as a boy.
He has long Black with grey streak in the middle of his head, with small black moustache. He is short as well.
- Image conscious, insists on carrying a Persian cat with him for the 'Celebrity Status' even though he is allergic to cats
- Insists on being addressed as 'Papa' instead of the more formal 'Chi Chi', presumably because it ties in better with his 'image'.
After the Yagyuu arc, Koshinori and his father Binbokusai both felt guilty about the years of pressuring Kyuubei into a boy decided to allow Kyuubei to do whatever she feels right.
- Based on Yagyu Munenori 柳生宗矩, a swordsman and founder of the Edo branch of the Yagyu Shinkage-ryu school.
- Male heirs in the family tend to be short.