She is the main character of the series. In the beginning of the series, Yumi is bashful beyond words, very timid and unsure of herself. She first met Sachiko, the star of Lillian School and had her tie fixed by Sachiko, which mortified Yumi. Yumi's friend, Tsutako Takeshima, took a cute picture of this and wanted to display it in the school festival. Tsutako got Yumi to ask permission from Sachiko. Other characters often comment on how her emotions 'show on her face'.
After Yumi first meets Sachiko, Sachiko is arguing with the Roses in the Rose Mansion. The Roses say that Sachiko cannot object to their plan, as she did not have a petite sœur. Sachiko runs out of the meeting and knocks Yumi over. She asks if Yumi has a grande sœur, and promptly offered Yumi her rosary. Sachiko thought that if she had a petite sœur, the Yamayurikai could not force her to play Cinderella alongside a male. Yumi refused Sachiko's rosary, as she would be accepting it for the wrong reasons. Sei Satō offers a bet to Sachiko—if she could convince Yumi to be her petite sœur, the role of Cinderella will go to Yumi. If she could not, she must play Cinderella. Sachiko tells Yumi that she, Sachiko, will definitely become Yumi's sœur. But deep inside Yumi, she feels terribly hurt by how nonchalantly Sachiko chose her randomly as her petite sœur; but despite this, she feels that precisely because she still likes Sachiko very much, she has "summoned the last vestiges of her pride to refuse Sachiko's rosary, and wishes Sachiko to at least understand how hurt she feels to be 'just a random pick'".
At a dress rehearsal for the play, it is revealed that the male who plays the prince, Suguru, is Sachiko's cousin and fiancé. Sachiko accuses him of going too far and runs away; Yumi follows her to the greenhouse. Yumi at first thinks that she cannot do anything for Sachiko, but Sachiko cries on her shoulder. Yumi offers to take the rosary, but Sachiko refuses. Sachiko tells Yumi about the arranged marriage, and that Suguru does not love her, but she loves him. After the play, there is a bonfire. Yumi thinks sadly that she would not have an excuse to have anything more to do with Sachiko, but suddenly Sachiko appears and offers Yumi her rosary; this time, Yumi accepts.
When the third years are about to graduate, Yumi asks Sei if she had anything to ask her, like taking care of the stray cat Goronta, or looking after Shimako. Sei says Yumi is the type of girl who will help even if not asked, so she is not worried about Shimako. Sei instead asked for a kiss on the mouth as a parting gift; Yumi tried to run away, but Sei hugged her. Sei thanked Yumi for helping her pass her examinations, since Yumi gave Sei a reason to live again. She also said that Yumi tried to run away so that it would not be Sei who was leaving her, but the other way around. Yumi realized that this is true. Yumi often gets hugged by Sei throughout the series, who thinks her to be very cute, which sometimes makes Sachiko angry due to jealousy. Yumi finds out from Shimako, however, that Sei would actually be going to the Lillian's Girls' University, which is in the same area as Yumi's school. She gets angry at Sei for not telling her, but is happy because Sei will not be leaving her after all.
