Yuki Mita is a wealthy, intelligent high school student and the narrative's primary antagonist. Acquainted with Fukaya since childhood, their history is defined by her habit of experimenting on him, leaving him with deeply negative memories. Her most persistent project is developing a perfect aphrodisiac specifically for Fukaya.

Driven by intense romantic feelings for him, Mita employs manipulative tactics to eliminate her rival, Fujino Shion. She conducts covert surveillance of Fukaya and Fujino, crafts a potent love potion, and tests it on herself—experiencing intimate illusions of Fukaya. She then deceitfully gives the potion to Fujino under the pretense of enhancing Fujino's relationship with Fukaya, while falsely warning against consuming it.

When the potion incapacitates Fujino, Mita approaches Fukaya, transferring the aphrodisiac via a kiss to trigger his heightened libido. Exploiting his altered state, she accompanies him home for non-consensual sex. She deliberately engineers Fujino's interruption to sow emotional discord between them.

After Fukaya rejects her to pursue Fujino, Mita reveals conflicting emotions: confessing her longstanding feelings and her prior virginity. When spurned, she falsely dismisses her emotions as insincere, claiming detachment as an "easygoing girl." This facade shatters as she later expresses anger and resolve to continue pursuing Fukaya. Her appearance features black hair and red eyes.