Kyūbei Yagyū, heir to the Yagyū family name, grew up under the clan's male-head tradition following her mother's post-birth death. Raised by her father Yagyū Koshinori and grandfather Yagyū Binbokusai, she endured childhood bullying from peers viewing her as an inadequate boy. During this time, she forged a pivotal bond with Shimura Tae, who defended her from tormentors. Kyūbei deeply admired Tae's fusion of strength and femininity, aspiring to emulate her. A defining childhood incident saw Kyūbei intervene when debt collectors assaulted Tae and her brother Shinpachi. Protecting them cost Kyūbei her left eye. Wracked with guilt, Tae vowed to become Kyūbei's "new left eye." This event ignited Kyūbei's resolve to surpass conventional strength standards—aiming to exceed any man or woman, even Tae herself. She departed for rigorous training, returning years later intending to marry Tae. Though her proposal failed after a dojo challenge loss to Shinpachi and allies, Kyūbei retained deep feelings for Tae. She remained fiercely protective in later arcs, rescuing Tae from execution during the Silver Soul arc and expressing willingness to die by her side. Initially presenting a cold, reserved demeanor, Kyūbei typically wore masculine attire: a white coat with red trim over a blue kimono, black eyepatch, and ponytail. Her upbringing instilled reflexive discomfort with male physical contact, causing her to throw men who touched her—a manifestation of androphobia. Exceptions emerged with Sakata Gintoki, whose touch occasionally prompted flustered or blushing reactions. This led her retainer Tōjō Ayumu to schematically encourage their interactions to "end her boyhood." Kyūbei's personality softened significantly after the Yagyū arc, revealing occasional feminine nuances. Throughout the Dekoboko Arc, she grappled with gender identity confusion stemming from her male-coded upbringing but ultimately rejected binary definitions, choosing to live solely as "Yagyū Kyūbei." When adopting feminine presentation, she wore twin ponytails, a flower-covered eyepatch, a pink kimono, and thigh-high socks. A master of the Yagyū Ryū swordsmanship style after intensive training, Kyūbei delivered lightning-fast strikes. Regarded among the clan's strongest practitioners, she outperformed elite fighters like Hijikata Tōshirō (while acknowledging his partial restraint). Overconfidence led to her defeat against Gintoki, who exploited her tactical inexperience. Her relationships include loyal service from Ayumu (who frequently pushed gothic lolita fashion despite her discomfort) and rivalry with Kondō Isao over Tae's attention. She shared cooperative moments with Tsukuyo during battles, where both reflected on accepting their womanhood. Her pet, Jugem Jugem, developed a profound bond with her. Gintoki earned her respect post-conflict, uniquely eliciting comedic or nervous reactions, with narrative hints suggesting potential romantic tension during group dates or amnesia-driven emotional moments.