Yukiji Ushiwaka is a second-year student at Katsuragi Zodiac Academy and president of its biology club. Her surname and physical traits, notably her ample bust, frequently draw comparisons to cows, suggesting a docile and gentle nature. She exudes a soothing presence that puts others at ease, yet displays consistent clumsiness—tripping unexpectedly or losing her way to school. This airheadedness leads her younger childhood friend, Mikine Nezu, to often care for her, inverting typical senior-junior dynamics.

Unintentionally suggestive remarks escape her without awareness of their implications, creating a perceived flirtatious or mature aura likened to a "married woman." A distinct quirk involves intense excitement triggered by the color red, a lifelong trait causing impulsive actions like fixating on red objects and physically overwhelming Mikine. Though diminished, this reaction persists in high school.

Her romantic history includes a past relationship with Saku Soma, exceeding friendship and involving physical intimacy. This complicates interactions with Saku's cousin, Aki Hiiragi, after Aki accidentally witnessed them post-intimacy. Yukiji openly disdains Aki's "good, healthy, and proper" nature, calling Aki her "least favorite type," cementing their tension.

As biology club president, Yukiji sidesteps conventional duties, and the club lacks standard activities. During a student council review, she preserved the club's status by submitting Mikine's personal observation diary—filled with embarrassing or unconventional entries about her—as fabricated club work.

Fuyuko Yuzui notes Yukiji observes Mikine and others in unusual ways. Yachiyo Inugami remains wary, viewing Yukiji as the sole person capable of provoking significant trouble through excessive teasing.