Yagyuu Kyuubei (柳生 九兵衛, Yagyū Kyūbei) is the successor to the Yagyuu family name, the daughter of Yagyuu Koshinori and the granddaughter of Yagyuu Binbokusai. She was brought up as a male, following the code of the Yagyuu clan, but in later years decided to follow neither femininity or masculinity, but live as only “Yagyuu Kyuubei”. She was the main antagonist of the Yagyuu Arc.

When Yagyuu Koshinori’s wife died right after giving birth to Kyuubei, he never remarried any woman. As the head of the Yagyuu family was traditionally always male and Koshinori was afraid Kyuubei could not properly back the family name as a woman, he decided to raise her as a man and samurai along with his father Yagyuu Binbokusai.