Yukiko Minase lives with her cousin Taki Minase as his legal guardian, though they consider themselves siblings. She serves as his primary caretaker, handling meals and household duties, though her part-time job and graduate studies often take her away from home. Academically, she holds a degree in French literature and pursued graduate work in the medical or biological sciences, fluent in French, English, and Latin. Her linguistic skills lead her to assist Taki in translating sections of the Bible Black grimoire, though she openly ridicules its perverse content. Yukiko possesses gold eyes and gray-greenish hair styled similarly to Taki's. She has large breasts, which she flaunts proudly, frequently wearing revealing outfits at home such as a white-and-blue suspender bra paired with light-blue mini shorts or leaving her clothing unbuttoned. Other noted attire includes a purple shirt tied with a blue ribbon, brown high heels, a red jacket with a black skirt, and cylindrical earrings. At school, she wears a lab coat over her outfit, accessorized with tan stockings and a garter belt. A distinctive silver necklace featuring a fish-shaped pendant is frequently visible. Personality-wise, Yukiko exhibits a strong-willed and teasing demeanor, especially towards Taki. She often enters his room without knocking and uses her sexuality to provoke him. She claims popularity with men but expresses frustration over lacking a boyfriend and remains a virgin. Despite her flirtatious behavior, she maintains a methodical approach to life, keeping her room meticulously clean. She is depicted as retaliatory when wronged. Her storyline begins with translating the grimoire and mocking Taki's interest. After Taki becomes corrupted by the grimoire's magic and aligns with antagonist Reika Kitami, he gropes Yukiko's breast and nipple, imbuing her with demonic arousal. This leaves her in a state of perpetual sexual craving, leading her to masturbate frantically in her room and later in her school's bathroom to relieve the uncontrollable urges. The effects escalate in the *Only* OVA, where Yukiko struggles with relentless arousal during class. After retreating to the bathroom to masturbate, she is summoned by a professor to his office. Despite initial reluctance, she engages in various fetishistic acts with him, including leash bondage and bodily fluid play, succumbing to her induced desires. This sequence occurs concurrently with the main series' events. Later, Yukiko discovers Taki's classmate Rika Shiraki in their home and is ambushed by robed followers of Kitami. She and Shiraki are subjected to repeated gang rape. Yukiko survives after Taki intervenes. Her final appearance shows her playfully pushing Taki out of their house when his friend Kurumi Imari arrives. In the game *Bible Black: The Infection*, Yukiko appears in a trance-like state during a school basement orgy, drawn by a mysterious force. She is rescued when Taki places a talisman in her mouth, rendering her unconscious for hospitalization. Earlier game narratives include alternate scenarios: one where school bullies rape her under the direction of corrupted classmate Kaori Saeki, and another where she jokingly wakes Taki by massaging his crotch with her foot. Her ultimate fate remains unresolved. While some narratives suggest memory alteration by characters like Hiroko Takashiro, other accounts imply Yukiko retains full recollection of the traumatic events but shows no lasting psychological impact. She appears only in the original series and its supplementary media.