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.