Calm and reserved, but usually with a smirk on her face, Yōko is Sachiko Ogasawara's grande sœur. As the nearly indisputable leader of the Yamayurikai during her time as Rosa Chinensis, she played a major role in maintaining stability in its sometimes unstable environment. Though usually cool and refined, she was also known to have a fun side. Additionally, it has been implied that she has an unrequited love for Sei Satou. She made Sachiko her petite sœur in a situation happened like Sachiko did to Yumi Fukuzawa. In Sachiko's second year, Yōko argues with Sachiko about the Cinderella play since she does not want to play with a male. Yumi suddenly comes in, and Yōko and the other two Roses offer them a bet. This eventually leads Yumi to become Sachiko's petite sœur. She does not appear much in the first season because of her exams. Before Valentine's Day, Minako Tsukiyama comes and asks the Yamayurikai if they can host the Card Hunting Game. Yōko says she wants the Roses Mansion to be one of the hunting locations so she can see the Roses Mansion filled with ordinary students before her graduation.
- At the beginning of the second season, Yōko comes to Sachiko's house on New Year's Day with the other Yamayurikai members, to join in a party there. Two days before her graduation ceremony, she comes up to Yumi from behind, giving her a surprise hug. Yumi asks if is she trying to make fun of her, but Yōko says that she wanted to hug her once before she leaves. She takes Yumi for a drink, telling the younger girl to take care of Sachiko.
- After the third year Roses graduate, only Sei ends up going to Lilian university; this makes Sachiko cry when she was chosen to give her onee-sama a goodbye speech at the ceremony. Yōko, at first, does not want to help her because that would imply that Sachiko is weak if she keeps relying on her onee-sama. When she cannot take anymore, she tries to help her, but Rei Hasekura goes to help Sachiko to say farewell to the third years. Seeing that her sœur has changed, Yōko is happy, saying that she can leave Lilian without any worries now. A few months later, when Sachiko's grandmother dies, Yōko is requested by her mother to meet Yumi. On their way to Sachiko's great grandmother's house, she explains everything to Yumi.
Youko is the current Rosa Chinensis during the 1st season of the series. During Youko's second year of high school, Rosa Gigantea and Rosa Chinensis (Sei and Youko's onee-samas respectively) tricked Yoko into taking an interest in Sachiko. Youko noted that Sachiko always walked and looked like she was fighting internally with something. With an endless desire to help (or meddle, as Sei put it) people, Yoko took Sachiko as her petite sceur. She had Sachiko quit her after school lessons (or in the novel, hinted about it) and once scolded her for not expressing her emotions and opinions. It was indicated in the novel that Yoko made Sachiko quit all her lessons so that her petite sceur will be able to relax.
Youko is an excellent source of logic and counsel to Sachiko Ogasawara, but knows her limits and realizes that Yumi Fukuzawa is the better person for providing Sachiko with affection, and reaching her heart.
She is known for being a meddler. She usually pushes people to do what is right and is supposed to be good for themselves. She is also an extremely good judge of character.
Youko is very fond of Satou Sei, although she doesn't openly admit it to Sei. Sei finds out just how much unrequited love Youko sent her way, but chooses not to reciprocate anyway.
Youko was the one who pushed Sei and Shimako to be soeurs. She told her soeur, Sachiko, that Shimako is quite crucial for the Yamayurikai and asked her to make Shimako her soeur. Youko thinks that it was the only way to push Sei into making up her mind and be Shimako's soeur.
Sachiko is her petite soeur. She made Sachiko quit her after school activities so that she will be able to relax. As also known for being a meddler, the Yamayurikai had an agreement of the play, Cinderella, that the prince will be characterized by the Handera School Council President, Kashiwagi. The said play actually has two objectives in involving Sachiko despite that it was a mere theater act to be hosted by the Yamayurikai. It's primary objective was to get rid of Sachiko's hatred of men while the secondary is to get Sachiko a soeur so that Youko will no longer worry about her anymore. In the end of the school festival along the play, the two objectives had been cleared whereas Sachiko faced her fear and gained Yumi as her soeur.
When Yumi was first introduced to the Yamayurikai, Youko was sure that Sachiko had just met her if not earlier. However, Tsutako's photo became an evidence that both Sachiko and Yumi had met before. Youko seems to be uncertain along with the Yamayurikai if Yumi is worthy to be a Rosa Chinensis en bouton petite soeur. Her inhibitions were verified wrong. Youko, with the Yamayurikai, were gladly surprised that Sachiko changed, though a bit at first, since Yumi had been her petite soeur.
Two days before her graduation ceremony, she comes up to Yumi from behind, giving her a surprise hug. Yumi asks if is she trying to make fun of her, but Yōko says that she wanted to hug her once before she leaves. She takes Yumi for a drink, telling the younger girl to take care of Sachiko.
- The name Youko means "light, sun, male" or "ocean" (you) and "child" (ko).
- Youko's surname Mizuno means "water" (mizu) and "field, wilderness" (no).
- It is implied that Youko has unrequited feelings for Sei Satou.
- After graduation, Youko pursued Law in her university years.