Kusuri Yakuzen is an eighteen-year-old student in her third year at Ohananomitsu High School, where she serves as the president and sole member of the Chemistry Club. She becomes the fifth girlfriend to the protagonist Rentarou Aijou. Her name carries a dual meaning: Kusuri is a homophone for the Japanese word meaning medicine or drug, and Yakuzen literally translates to medicinal cooking, a fitting label for a character whose life revolves around chemical concoctions.

Physically, Kusuri alternates between two forms as a direct result of her own experiments. She originally created and ingested an immortality drug, but the formula was flawed. Instead of granting eternal life, it left her body frozen in the shape of an eight-year-old child for most of the time. In this child form she has shorter red hair, does not require glasses, and dons an oversized lab coat alongside her school uniform. She can temporarily revert to her actual teenage appearance, though only for limited durations and at a physical cost. In her eighteen-year-old body, she possesses long, wavy red hair tied in a ponytail, turquoise eyes, glasses, a slender build, and a distinctive beauty mark on the left side of her chest. This dual nature defines much of her daily life and interactions.

Her personality is energetic, playful, and deeply inquisitive. Kusuri displays a cheerful disregard for potential side effects or disasters tied to her inventions. She approaches chemistry with unrestrained enthusiasm, viewing everyone around her as willing test subjects and rarely pausing to consider safety before dosing herself or others. Her speech is peppered with the verbal tic nanoda at the end of her sentences, and she often refers to herself in the third person, reinforcing an almost childlike self-image even when discussing complex scientific concepts.

As a prodigious scientist, Kusuri’s primary ability is drug creation on a near-limitless scale. She is capable of synthesizing substances that can alter physical traits, emotional states, sensory perceptions, and even fundamental biology—drugs that can turn people into babies, swap consciousness, induce temporary superhuman abilities, or create hallucinatory experiences. Her lab is the narrative catalyst for many chaotic scenarios, effectively positioning her as the cast’s chemistry cheat character who can produce a plot-driving formula for almost any situation. This talent stems from raw ingenuity rather than formal caution; she invents freely and impulsively.

Within the story, Kusuri’s introduction expands the found-family dynamic of the girlfriend group. She was the first older girlfriend Rentarou encountered after the initial four, and her unique situation—appearing as a child while being chronologically older—adds a comedic and sometimes protective layer to her relationships. She gets along warmly with the other girlfriends, both as a playmate in her child form and as a dependable (if unpredictable) ally in her teenage form. It is later revealed that both of her parents and her grandmother also consumed the same flawed immortality drug, so her entire immediate family shares the same childlike appearance, making her background a hereditary quirk of shared scientific ambition.

Her motivations are rooted in an unceasing desire to experiment and create. She is driven less by grand goals than by the sheer joy of discovery and the fun of seeing her concoctions upend daily life. While she does mature in small ways—occasionally learning to weigh the consequences of her actions or collaborating more responsibly with Rentarou and the others—her core remains that of a brilliant, mischievous inventor who finds love and belonging in a household that accepts her chaotic genius without needing her to truly grow up.