Riri and Karin are the only ones for me. The reason, I guess, is that touches and kisses are my favorite part of a relationship. A girl who only wants to look at me, or listen to me, or (god help us) sniff at me all day long is just... not my idea of a desirable partner.
I had a slightly greater liking for Riri before, but Karin has gotten so much screen time lately and has grown so much as a character that it's difficult not to be rooting for her now. She seriously and sincerely proposed to Mei. She asked Mei to get into a committed relationship with her. Mei gave an ambiguous "I like you but I'm not ready" sort of reply, so now Karin is giving her time while trying to get them to know each other better and become closer. And as she does this, because she's a reasonable girl, she doesn't expect Mei to forswear flirting with other women (since they haven't actually committed to each other yet) and of course she doesn't hold back either from doing the same thing.
And this leads us to the scene in this chapter where she makes out with that cute kohai girl and finds out, after kissing her, that it wasn't all that enjoyable... because she has kissed Mei before; and this, she realizes, has made her lose interest in other girls' lips. Because it's just not the same. In other words, kissing Mei to her was like:
Tbh this is so incredibly ROMANTIC it has totally swept me off my feet and now all I can think of is that I want Mei to stop being a dummy and accept Karin's feelings. (Sorry, Riri-chan, I still like you, but... >_< ) Heck, I mean, come on, that line about rejecting Karin because she feels she's not good enough for her? That's the oldest cliché in the book of romance tropes, and it was as dumb the first time it was ever used as it is today!!! For gossake, Mei, if she says you're the right one then you're the right one, stop overthinking things!!!!!!