Being Harumi's sister, she bears resemblance to Harumi. Like Harumi, she's really tall. She has dark purple hair styled in a bowl cut reaching the height of her chin, with a longer bang on the right side of her face just long enough to touch her shoulder. Back in her Aihara Academy days, she had longer hair, which she kept in a ponytail.
Mitsuko is a cold and serious person, similar to Mei. However, unlike Mei who can be concessive, she firmly believes that the school's rules should be kept at all costs. According to Harumi, during her time as the student council president, she ruled the school with an iron hand.
When Mitsuko was still the student council president in Aihara Academy, Mei apparently was the president of the Branch School and reported to Mitsuko. Mitsuko had high hopes on Mei as High School's student council president, and trusted Maruta, back then a first year student, with both the school and the next student council president.
She first appears after Harumi returns from the trip, asking Harumi about rumors of a blonde student in Aihara Academy. She decides she will participate in the Student Council's next meeting.
At the start of the next school year, Harumi tells Yuzu about her sister, and warns her that if her sister comes to school, she'll "beat [Yuzu] up good". Soon after, Mitsuko visits the academy. Finding Nene Nomura having a talk with the Student Council, she scolds the freshman, and also scolds Mei and Himeko for not doing their work. Just then, Yuzu comes to defend Nene, bringing Harumi along, and removes her wig in defiance to Mitsuko. Mitsuko, then, decides to force her into an election race against Mei for the Student Council presidency.
Later, Mitsuko attends school to see the candidates' speeches, scaring two teachers sitting near her. She surprises everyone by applauding to Yuzu's speech after she finishes. She then tells the teachers that there's a big difference between simply receiving something and accepting it with your own determination, which confuses them, and that being able to see this difference helps someone create their own path. She leaves during Mei's speech, after Mei states she will add changes to the school's rules.
During a conversation Yuzu and Harumi have, Harumi mentions her conversations with Mitsuko are becoming less apprehensive since the elections, and Mitsuko is making her see things more from a sister's point of view.
On a latter date, Mitsuko and Harumi go grocery shopping together, with Harumi apprehensive that Mei might find her there and discover Yuzu is not actually having study sessions with her, while Mitsuko says she and their grandmother eat too much sōmen everyday and that alone won't give them resistance during summer. Harumi asks if they can't finish shopping and go back home already. Mitsuko asks why, and Harumi, unable to tell why, says it's inconvenient for her to stay out of home for various reasons. Mitsuko then says they'll have Kimchi Nabe, so they can just pick up the ingredients and head back home. Harumi is relieved, realizes Mei is in the grocery store, and so sends her sister another way so they won't meet, but they do either way. After Mei greets her, Mitsuko asks her if they're having Kimchi Nabe in their place too, but Mei says she's thinking of spicy dishes, to which Mitsuko suggests the Kimchi Nabe she already mentioned. Mei then calls Harumi's attention, apologizing for any trouble Yuzu may have caused to her. Mitsuko then gets angry with Harumi for not telling her Yuzu was home when she was there, saying it's rude not to greet her, and then Mei asks if Yuzu wasn't home, so Harumi tries to explain that Yuzu had to leave.
Harumi is Mitsuko younger sister. Mitsuko does care for her and wants what's best for her. Unfortunately, because of her cold personality, Harumi keeps her distance, which does bother her. She does truly care about her sister and wants her to be happy and grow up her own way.
not much is known about them though they seem to be on friendly terms with one another.
Mitsuko is very harsh to Mei, as seen when she scolded her for not keeping the students in line. She does respect her and left the school to her in full confidence and was very pleased with her how she chose to run the school.
Himiko seem to be afraid of her and due to that fear was worried about Yuzu and how things would turn out.
Apon first meeting one another Yuzu was defending Nene from Her and she didn't seem to like her but after her speach was pleased to see her speaking her mind and standing up forherself and other students.
- "Mitsuko" can have different meanings depending on the writing, such as 光子 ("shining child"?) or 睦子 ("harmonious child"?). Mitsuko's name, however, has no actual meaning as it's written in Hiragana, although "子" is the kanji for "child". "Taniguchi" (谷口?) means "mouth of a valley".
- Based on Harumi's description, Yuzu imagined Mitsuko as a Harumi look-a-like dressed like a stereotypical Japanese delinquent and holding a club with nails running through it.