Tsubaki, aged 24, is Makoto's second eldest sister and a biology teacher. She arrives at Makoto's and Ito's school under her father's orders to keep an eye on him. Among her siblings, she is the one who most diligently follows their father's wishes. After Sakura left, Tsubaki managed to lessen her father's influence over the household, leading to some resentment towards her elder sister.
Tsubaki is infatuated with Makoto, covering her room walls with pictures of him. She aims to bring him back home, convinced that she knows what is best for him as his older sister. In his younger years, Makoto was very close to her, calling her "Tsubaki-chan," and it's evident she still cherishes memories of him as a little boy. Oddly, she occasionally sends Makoto frilly dresses, despite disapproving of his bet with their father.
She is a firm and straightforward person with a remarkable leadership ability, exemplified by how she trains her class to excel in athletics for the inter-class tournament. Tsubaki has a shota complex and was disappointed to learn that Ito is a girl.
After accidentally discovering the relationship between Makoto and Ito, Tsubaki orders Makoto to return home, citing the broken deal with their father. However, she relents at Makoto's pleading, agreeing to back off if Makoto and Ito's class wins the inter-class athletic competition, which they narrowly lose. With Akane's help, they recreate a scene from Beauty and the Beast for Tsubaki, persuading her of Makoto's genuine passion for acting and his deep love for Ito. She finally promises not to tell their father and eventually leaves the school.
Tsubaki contrasts with her older sister Sakura, who has no interest in pretty boys. The name "Tsubaki" means "camellia."
