Kusuri Yakuzen is a main character and the fifth girlfriend of Rentarou Aijou. She is a third-year student at Ohananomitsu High School, attending class 3-3, and serves as the president of the chemistry club. Kusuri is a genius chemist, capable of creating drugs and medicines with a vast array of effects, ranging from miraculous to catastrophic. Her abilities are considered world-class, to the point where pharmaceutical spies have targeted her inventions. Despite her prodigious talent, she is quite careless in her experiments, often leading to unintended and chaotic results.

The most defining characteristic of Kusuri is her unusual appearance, which is a direct result of her own work. She once consumed a failed immortality drug she had been developing. The side effect permanently altered her body, reverting her physical form to that of a child around eight years old. In this state, her hair is short, she has two prominent locks of hair standing up, and her personality becomes lively, energetic, and somewhat immature. Her mental state is also affected, making her behave much younger than her true age. When she needs to return to her original body, she drinks a special cancellation drug. In her true adult form, she is tall and voluptuous with long, wavy red hair tied in a ponytail, turquoise eyes, and she requires glasses. Her personality in this form is more mature and composed, though she retains her core enthusiasm. Kusuri typically stays in her child form because her adult figure can be physically obstructive during chemistry work.

Kusuri has a distinctive verbal tic, frequently ending her sentences with nano da, translated as yep yep, and laughs with a sound like kusuri kusuri. She is highly ingenuitive and obsessed with her craft, often forcing her experimental drugs on herself or others without concern for the potential dangers. A peculiar habit born from her intense focus is that she always wears a diaper under her clothes, finding bathroom breaks to be an annoying interruption to her experiments. She is also a fan of the children's anime Pepepe no Pen-tarou, adoring the main character because he resembles Rentarou.

Her role in the story is that of a chaotic inventor. Her drugs are a recurring plot device, used to solve major problems, such as during the mission to rescue Hahari Hanazono, but they are just as likely to create new, bizarre predicaments for the Rentarou Family. Her creations are the catalyst for many comedic arcs and adventures.

Kusuri comes from a family of eccentric chemists, all of whom have similarly been affected by her immortality drug. This includes her mother, her father, and her grandmother, Yaku Yakuzen. Kusuri has a deep love and admiration for her grandmother. Her primary motivation for developing the immortality drug in the first place was to prevent her beloved grandmother from dying of old age. While she once dreamed of joining an overseas pharmaceutical research team, after meeting Rentarou and becoming part of his ever-growing family, she finds greater happiness in using her talents to live alongside them. Her personal development is marked by this shift from a solitary scientific ambition to finding fulfillment in shared, chaotic daily life.

Kusuri shares a close bond with her grandmother Yaku, who later also becomes one of Rentarou's girlfriends. The two have similar childlike appearances and often spend time together, watching sumo or their favorite anime. In her child form, she frequently teams up with the small and quiet Shizuka Yoshimoto, with the two forming a duo of mascot-like characters. She is also classmates with Torotoro Kishika. Kusuri's relationships are central to her character, as her love for her family, particularly her grandmother, is the driving force behind her most significant creation.