Yuri Project
joined Jul 14, 2016
In the earlier chapter where Ririka was super pissed that Yuzumori was having fun with her classmates instead of being lonely and depressed, I think at that point it was already hinting heavily that Ririka wanted to monopolize her, or at least wanted Yuzumori to have no one else. It's consistent with all her earlier behavior too, so the author did a fine job setting this up.
I don't really see Ririka as a villain here. For one thing, look how spectacularly easy it was for her to break up Yuzumori and Mimika. Yuzumori started acting distant as soon as she saw Ririka and Mimika together, before she'd even said or done anything, and in the end she really didn't do much since their relationship was already so fragile in the first place.
The other reason is that Ririka's bullshit reason for Mimika to stay away was actually true—Yuzumori got closer to her classmates and started to make friends. I actually really liked that bit, since it plays up the inherent contradiction in this series. On the one hand, it's a story about a romance between a 10 year-old girl and a high school student, but it also has to acknowledge that, pure-hearted though it might be, such a relationship is generally unhealthy and detrimental.