Yōko Mizuno is one of the main characters in the yuri series Maria-sama ga Miteru. Calm and reserved, but usually with a smirk on her face, Yōko is Sachiko'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. She have been in Plum class during her first year of high school at Lillian Academy. She have been in Camellia class during her third year of high school at Lillian Academy.
- In the Rose Mansion, where the heads of the Yamayurikai have been meeting there every day to prepare for the school festival, she invited Yumi to come to the Rose Mansion anytime because she's Sachiko's seour, that mean she accept Sachiko's choice, for Sachiko has already decided, then she won't debate.
- The thing she wanted to confirm was (what Yumi is to Sachiko), and if she still thinks of Yumi as her seour, she even encouraged her to give her the Rosary while still being in the Rose Mansion.
- She made it so Sachiko was forced to quit her world, because she made her her soeur and dragged her into doing all sorts of Yamayuri Council chores, for Sachiko is an earnest person at heart, so when she's faced with expectations, she does exactly that, that's why she gave her the break she needed once in a while.
- She started the review session with a toas of tea in commemoration of Sachiko officially making a conquest with Yumi.
- Additionally, it has been implied that she has an unrequited love for Sei Satō.
- The time when she was Rosa Chinesis en bouton she stipulated Sei if she wanted to make Shiori her little sister, to formally present her rosary and introduce Shiori to everyone properly.
- After she entered the Rose Mansion she realized Sachiko's countenance has improved since making Yumi her soeur, then winked at Yumi.
- The reason she won't explain the contents of the girls' love novel "The Forest of Briars", neither Sei's past to the petite soeurs is because she wants to respect Sei's feelings, it goes to show just how painful the incident Sei must have been when Shiori Kubo was attending Lillian Academy.
- Sneaking behind Yumi and hugging her, she assumed she missed her chance to fully enjoy the experience of cuddling Yumi, for she just wanted to try it once, the unique experience of being ʺA pseudo-serial huggerʺ with her, since she can't do it anymore once she graduate, and it's not as if she's going to the affiliate college.
- After she invited Yumi along to the milk bar, the reason she told her for why she think very highly of her is because she's very perceptive and also interesting, not as someone who had experience with hot strawberry milk au lait, but as Sachiko's guardian, wherefore she conveyed her ʺwillʺ to Yumi placing her to take care of Sachiko in her place, she described Sachiko as her dearly beloved little sister.
- When Sachiko started to cry during her speech of the graduation ceremony of the Lady Roses, she wanted to go to her instantly, but her only obstacle was that she was afraid that people will come to think Sachiko can't do anything without her big sister, she couldn't watch anymore so she advanced to reach the stage not until Rei showed up and saved the situation with completing the speech.
- Because Sei made a big fuss of fury at her entrance to the room where she was persuading Shimako to help at the Yamayuri student council, she assumed Shimako was a significant person to Sei, even though Sei had never even given it any thought, but Youko had the ability to perceive what was in Sei's heart even more accurately than she could as always.
- Her regret: she wished to force Sei and Shiori to become soeurs, that way Sei may have been able to make some good memories at least for the two years that she was in Lillian Academy, though she's not thinking of Shimako as a replacement for Shiori.
- When Sei blurted out the name Shimako Toudou, she believed she had the same expression on her face as when she first mentioned Shiori Kubo, and having actually seen Shimako, she realized something, Shimako is similar to Sei, yet she refrained from saying exactly how, because if she do, Sei might slap her for real that time, but she gave a hint about how they're similar has to do with their weaknesses.
- She noted that she love even the weakness in Sei.
- She is glad to have the girl who has long been a fan of hers pin a white rose on her chest with flowing tears for the graduation ceremony in Camellia class.
- The reason why she approached Sei on their first year of high school at Lillian Academy was because she found herself interested in her somehow.