Kusuri Yakuzen is a fictional character from the manga and anime series “The 100 Girlfriends Who Really, Really, Really, Really, Really Love You,” where she appears as the fifth “fated girlfriend” of the protagonist Rentarou Aijou.
She is a third-year student at Ohana no Mitsu University Affiliated High School, an eccentric genius chemist, and the president of the chemistry club, famous in-story for her outrageous drug inventions and her dual child/adult forms.
Full Name: Kusuri Yakuzen
Gender: Female
Age: 17 (at the time of Rentarou’s entrance ceremony), transforms into her original 18-year-old form temporarily
School: Ohana no Mitsu University Affiliated High School, Class 3–3
Role: Chemistry Club President
Birthday: April 18
Zodiac Sign: Aries
First Appearance (manga): Volume 2, Chapter often referred to as “Bibiiin!!”
First Appearance (anime): Season 2, Episode 7 (corresponding to Volume 2, Episode 10)
Rentarou Family “joining” episodes (anime): Season 2, Episodes 7–8 (manga Volume 2, Chapters 11–13 range)
Theme BGM: “Kagakubu no Kusuri Yakuzen nano da!” and “Kusuri’s True Form nano da yo.”
Image Song: “Pen-tarou’s Big! Big! Big Adventure”
Voice Actors: Hisako Kanemoto (voice drama), Ayaka Asai (TV anime)
Personality Markers:
First-person pronoun: “Kusuri”
Child form sentence ending: “nano da”
Adult form sentence ending: “nano da yo”
Laughing onomatopoeia: “kusuri kusuri” (a pun on “medicine” and giggling)
Family:
Grandmother: Yaku Yakuzen (also one of Rentarou’s girlfriends, the 16th)
Parents: unnamed in this text, but also part of the same medicine-obsessed family line
Favorite Show: “Pepepe no Pen-tarou,” a children’s anime (she especially adores Pen-tarou because he resembles Rentarou).
Club: Chemistry Club (president; essentially the only remaining member due to past incidents).
Main Hobby: Developing all kinds of medicine-like “drugs” and experimental concoctions.
Special Physical Feature: A mole on her left breast (visible in her adult form).
Kusuri Yakuzen is Rentarou Aijou’s fifth fated girlfriend and his first older-girlfriend-type, at least at the time she appears.
Her role in the story is that of a “chemistry cheat character,” constantly creating new drugs whose absurd effects launch many episodes and arcs.
She comes from a family that loves drug development to a comical degree, and she is a born chemist who has been playing with distillation equipment since childhood instead of normal toys.
It is said that she developed a new drug at eight months old, and her inventions are described as being so impressive that even Nobel prize winners would be shocked.
Originally, Kusuri dreamed of joining an overseas pharmaceutical research team.
However, after meeting Rentarou and the ever-growing “Rentarou Family,” she discovers that what truly makes her happy is developing drugs while living together with them.
Because her abilities are unique and world-class, pharmaceutical spies sometimes target her.
Each time, her father drives those spies away, allowing Kusuri to keep living her chaotic but fun school life.
Although she technically holds several “senior” positions (first older girlfriend, high school third year, etc.), the cast later includes even older girlfriends, college students, and even a repeat senior year student.
As a result, her “older girl” status fades in importance, and she is primarily recognized as “the mad chemist character.”
Childlike Eight-Year-Old Form
Most of the time, Kusuri appears as a tiny, childlike girl, roughly the size of fellow small character Shizuka Yoshimoto.
She has short red hair and two prominent ahoge strands sticking up at the top of her head, compared to the tail of a squirrel.
Her usual outfit is the standard school uniform with a suspender skirt, over which she wears an extremely long lab coat that reaches the floor.
In this form, she looks nothing like a third-year high school student and far more like a grade-schooler.
This appearance, however, is not her natural body: it is the side effect of a failed “immortality drug” she herself developed and drank at some earlier point.
As a result, her body is reverted to around eight years old, and her mental state is also pulled down to around that level, though she still thinks and reacts more like a middle schooler than an actual eight-year-old.
In child form, she is physically agile enough to swim, play baseball, and join athletics practice, although she lacks training compared to the sports clubs.
Her classmates treat her like a cute class mascot, and she often teams up with Shizuka as a “twin loli mascot” duo in early chapters.
She constantly wears a diaper under her clothes.
The reason is purely practical: when she gets absorbed in experiments, she finds going to the bathroom “annoying,” so she preps for the worst and sometimes actually has accidents.
Adult Eighteen-Year-Old Form
Her true body is that of an 18-year-old high school senior: tall, long-haired, curvy—roughly matching Hakari Hanazono in glamorous proportions.
To access this form, she drinks a special “cancelling drug” that temporarily negates the immortality-drug’s side effect.
In adult form, her height and hair length increase, her figure becomes very voluptuous, and her mental age catches up to a more typical 18-year-old.
Her speech also shifts; her sentence ending changes from “nano da” t
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