Kyuubei is the sole heir to the Yagyuu family, being the daughter of Yagyuu Koshinori and granddaughter of Yagyuu Binbokusai. She is brought up as a male, following the code of a samurai. She is a genius swordsman; surpassing average swordsmen in skills, and is able to hold herself against veteran swordsmen, like Hijikata Toushirou and Sakata Gintoki.
During her introduction, she tried to take Shimura Tae as her fiancee, though Shimura Shinpachi and the rest stopped her from doing it. She still has feelings for Tae, though they remained in friendly ties. She usually follows wherever Tae goes and does the same things that Tae is doing. She also protects Tae from the stalker Kondou Isao.
She is always being guarded by Toujou Ayumu, to the point of him being overprotective towards her. Ayumu also tries to convince her to wear gothic lolita outfits, which cause irritation to her, blasting/beating him up because of it.
In recent Gintama chapters, she became the target of jealousy of Katsura Kotarou, as he felt that she is recently becoming more active in the manga spotlight than him.
Kyuubei has a bit of a cold personality, probably due to how she trained herself to become strong. However, Tae's influence is currently shaping a more softer Kyuubei, as they share more time together during their free time.
She also has an automatic reflex against men who touch her or she touches (though she did rescue Gin one time by catching his hand).
In episode 190 it's revealed that, apparently, she really likes Otae's cooking.
Her name is probably referring to the famous samurai Yagyu Jubei Mitsuyoshi, due to "Ju" meaning "Ten" and "Kyu" meaning "Nine."
