When Yagyuu Koshinori’s wife died right after giving birth to Kyuubei, he never remarried and raised Kyuubei as a strong samurai along with his father Yagyuu Binbokusai. 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.
This caused children to bully Kyuubei during her childhood. At the same time she met Shimura Tae, who beat up the ones who made fun of her and became an important part of her life. She admired Tae’s strong yet feminine personality and wanted to be just like her. When debt collectors physically abused Tae and her brother, Kyuubei attacked them to protect her. Kyuubei lost her left eye in the process, which filled Tae with guilt. She swore to Kyuubei that she’d become her new left eye.
Kyuubei later left with the ambition to become stronger than a man or a woman and even stronger than Tae. She underwent a rigorous training to achieve her goal, and later returned with the intention of marrying Tae.