Kei Yūki is a first-year college student at an agricultural university in Tokyo, having moved from a rural farming community to the bustling city to pursue a higher education. Kei comes from a family that runs a traditional sake brewery, which is the source of both deep knowledge and a significant personal trauma. Years before the story begins, a chief brewer at the family's facility, who had resisted modern sanitation methods, caused a large batch of sake to be ruined by a hiochi bacteria infestation. Overcome with shame, the man committed suicide, and a young Kei discovered the body hanging in the brewing cellar. This event left a lasting psychological scar, leading Kei to view hiochi bacteria as a personal enemy and to develop a ritualistic, almost solemn determination when eradicating them.

Initially, Kei is presented as a calm, sensible, and observant individual who often serves as a grounded foil to other, more eccentric characters. Kei is sharp-witted and practical, able to think quickly to deflect awkward questions or notice small details in the environment. While generally even-tempered and not easily swayed by others' opinions, Kei has moments of spontaneous and loud reactions, particularly when confronted with the intensely fermented and foul-smelling foods that the university lab is known for. A significant facet of Kei's personality involves a deep-seated discomfort with being perceived as feminine, a trait that was a source of distress and anger throughout childhood. This internal conflict, described as a reaction-formation against a hidden truth, forms the core of a major personal transformation.

After taking a leave of absence from the university, Kei goes to work at a liquor store owned by a friend of a professor. During this time, Kei reappears having adopted the fashion style of an Elegant Gothic Lolita, complete with a wig and makeup. This external change is accompanied by a shift in mannerisms toward greater poise and femininity, marking the beginning of a journey of self-acceptance regarding a long-suppressed gender identity. In this new presentation, Kei remains knowledgeable, cheerful, and a reliable confidant but is now actively trying to live more authentically, charging forward despite an acute awareness of how others might react. This development is a key part of Kei's role in the story, which is to learn about oneself and to live life fully by not discarding possibilities.

Kei's primary role in the narrative is as the childhood best friend of the protagonist, Tadayasu Sawaki. The two grew up together in the countryside and entered the agricultural university as a pair, providing a sense of familiarity for Sawaki. Kei is often a sympathetic listener and a source of wisdom for Sawaki, especially when the latter struggles with the implications of his unique ability to see microbes. Their relationship is intensely close, and Kei's feelings for Sawaki go beyond simple friendship, though the exact nature of those feelings is a complex and recurring theme. This dynamic leads to several awkward and comedic situations, including a moment where Kei impulsively kisses Sawaki. Despite any romantic tension, Sawaki declares that they will always be inseparable friends, solidifying their bond as a form of platonic life partnership.

Motivated by the tragic incident at the family brewery, a key goal for Kei is to gain mastery over the sake brewing process and learn how to definitively eliminate the threat of hiochi bacteria. This professional motivation is paired with a personal quest for self-discovery, using the freedom of college life to try new things and ultimately come to terms with gender identity. Kei's notable abilities are grounded in practical knowledge rather than superhuman powers. Possessing extensive expertise in the sake brewing process, Kei is also knowledgeable about various agricultural products and fermentation methods. This practical background is complemented by experience working in a bar and a brief but informative crash course in using an electron microscope.