Hailing from an ordinary shinobi family, Yagyū shared an inseparable bond with her younger sister Nozomi. After a car accident claimed Nozomi's life, Yagyū fashioned an eyepatch from her sister's ribbon, wearing it over her functional right eye as a constant tribute that deliberately limited her vision to honor Nozomi's memory. This loss initially drove her into solitude until she enrolled at Hanzō National Academy.

There, Yagyū encountered Hibari, whose uncanny resemblance to Nozomi deeply affected her. Hibari's cheerful nature gradually drew Yagyū out of isolation, compelling her to adopt a fiercely protective, sisterly role. This devotion evolved into a "sister complex," manifesting as Yagyū prioritizing Hibari's safety above all else—rushing to her aid instinctively. While strengthening their bond, this protectiveness sometimes led her to underestimate Hibari's abilities or trigger unintended outcomes.

In the spin-off *Bon Appétit!*, Yagyū participated in a cooking tournament alongside Hibari upon learning the grand prize was a wish-granting scroll, initially hoping to deepen their connection. She defeated opponents including Asuka and Daidōji, whose encounters with her nurturing spirit prompted them to address her as "Sis." Facing Hibari in the finals filled Yagyū with dread at the thought of "breaking her heart" through victory. After winning, she comforted Hibari and tore up the scroll, realizing their authentic bond had already fulfilled her wish without supernatural aid.

Yagyū possesses long white hair styled in twin-tails secured by shuriken-shaped holders, crimson eyes, and her signature eyepatch. She stands 158 cm tall with measurements of 85-60-83 cm. Her primary weapon is a bladed oilpaper umbrella, and her guardian animal is the squid. Typically stoic and reserved, she exhibits subtle quirks like eating dried squid without utensils. While generally distant, interactions involving Hibari can provoke rare emotional displays.