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.
At a dress rehersal 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.
After the third years graduate, spring comes, along with the arrival of Tōko, Sachiko’s cousin. Yumi feels jealous of Tōko’s closeness with Sachiko, but does not say anything about it. Yumi later finds out the truth about Sachiko’s grandmother dying. At the day of the funeral, Sachiko apologizes and Yumi says that she loved her onee-sama very much.
At the beginning of the OVAs, Sachiko invited Yumi to her summer home for a vacation. Sachiko’s acquaintances make trouble for them. They then invite Yumi and Sachiko to a party which later turns out to be Sachiko’s great grandmother eightieth birthday party. The acquaintances ask Yumi to play a musical instrument. Yumi says that she is no good at playing anything, but sings the song “Maria-sama no Kokoro”. Sachiko accompanies her with a piano performance. After that, Sachiko’s grandmother congratulates Yumi, because the song brings back memories. She calls Yumi a cute angel, and asks her to visit next year.
In the first season, Yumi is Rosa Chinensis en bouton petite soeur, her grande soeur being Ogasawara Sachiko.