Mihari Oyama, the younger sister of Mahiro Oyama, entered the world seventeen years before the series as the Oyama family’s second child. From a young age, she displayed exceptional academic and athletic prowess. During elementary school thunderstorms, Mahiro comforted her by wrapping them together in a blanket, forging their early bond. At seven, he won her an Oshima stuffed animal at an amusement park, which she cherished. Enrolling at Yotsuba University Affiliated Junior High School at thirteen, she befriended Kaede Hozuki and received a gold hairclip from Mahiro as a reward for stellar test results, making it her most treasured accessory. Her brilliance inadvertently fueled Mahiro’s inferiority complex, leading to his isolation. Concerned about his reclusive life and inspired by Kaede’s remark on fashion boosting confidence, Mihari devised the "Onii-chan Rehabilitation Project." She bypassed high school via advanced placement exams to enter Yotsuba University, specializing in biology and chemistry to create a drug transforming Mahiro into a middle school-aged girl. Her faculty advisor, Chitose Azuma, aided in perfecting the formula with undisclosed motives. Mihari administered the drug to Mahiro without consent, triggering his transformation. Physically, Mihari stands 154 cm tall with dark brown hair in pigtails tied by red bows, featuring purple streaks in the anime. She consistently wears a red button-up top, brown skirt, brown belt, gold medallion necklace, and white lab coat across settings, switching to short sleeves in warm weather. Though she outgrew her middle school uniform, she can still fit into it, reflecting limited physical growth since then. Sensitive about her bust size during comparisons, she secretly tested her transformation drug to enhance her own figure. Mihari possesses high intelligence and athleticism, holding unbroken track and field records from middle school. She manages all household duties—cooking, finances—due to her parents working overseas. Despite her competence, she lacks fashion confidence, often layering impractical combinations like sweatpants under skirts for warmth. She delights in choosing cute outfits for Mahiro but feels too embarrassed to wear them herself. Emotionally, she tears up when moved by Mahiro’s kindness or growth. Her severe alcohol intolerance causes immediate intoxication from minimal exposure, prompting impulsive kissing of nearby females—incidents involving Kaede, Miyo Murosaki, and labmates Arika, Tsubasa, and Chitose led to Kaede imposing a permanent drinking ban. Mihari’s core drive is her profound care for Mahiro, aiming to reintegrate him into society. She monitors his progress, enforcing rules like regular showers and school attendance while providing financial support. She nurtured his friendships by inviting Momiji Hozuki and others home, trusting them to support him at school. Their dynamic evolved from distant siblings to a closer bond, with Mihari assuming an older sister role. She finds joy when Mahiro reciprocates care—during her illness or when acknowledging her efforts—and an accidental body-swap deepened their mutual understanding. Her friendship with Kaede began in middle school, where Kaede taught her cooking and fashion; they remain close, with Kaede often assisting domestically. Mihari interacts with Momiji, Asahi Ōka, and Miyo through Mahiro, earning the nickname "Professor" from Asahi despite lacking a doctorate. She knows Nayuta Tenkuta from the university lab but rarely engages her. Mihari shares a fear of thunderstorms with Kaede, rooted in childhood trauma eased by Mahiro.