Sakaki Rio, Yuki’s tall, cool, athletic new rival, probably thinks the world is out of alignment, and she simply needs to click it back into a place where she, not Yuki, is by Momo’s side, “protecting his smile”, as anime characters love to do.
That Yuki simply slipped into Momo’s good graces with her Poodle-Maltese-esque cuteness and outwardly easygoing nature; but she doesn’t deserve Momo; not really. He’s Rio’s small, cute animal to protect.
But at the start of the episode, she’s at a distinct disadvantage. She’s in the supermarket. Yuki ends up in Momo’s room, then bed, then enjoys dinner with him. Definite FOMO vibes coming off Rio.
It wasn’t always that way. She joined the tennis club because Momo thought she’d look cool playing. Then he quit, and she got stuck practicing with the older girls that craved her athletic prowess.
She lost her “in” with him, though she’s still in the same class as him and accepts his offer of ice cream in exchange for her study cards. But it’s clear Momo doesn’t see her at all the way Rio sees him, which is more how he sees Yuki. To Momo, she’s just…a friend.
Whereas Rio’s effort to get closer to Momo (thru tennis) fizzled out, Yuki has gotten to where she is through relentless effort and persistence, without ever exposing too much of her perv-ier side to Momo. More to the point, he’s all but blind to that side of her-like assuming she was feeling ill when he found her in his bed.
What’s amusing about these two is that Momo is often worried that he’s thinking impure thoughts or being pervy and tries to check himself, while Yuki leaps into that arena with such gusto, she often needs her friend to pull her out, by, say, tossing out Momo’s used straws.
When she inevitably gets found out by Rio, Rio doesn’t turn it into leverage, because she can’t, not without revealing her own hand to Momo (or worse, coming off as mean or vindictive). She’s also clearly a little afraid of Momo’s intense observation and intel-gathering methods.
But Rio also knows that there’s more to Yuki than she thought …much more, though not all of it necessarily good. Yuki is right that Rio understands how she feels about the addictively-cute Momo, but the sheer intensity of Yuki’s obsession overpowers any resultant shame. Rio just isn’t wired that way, but they both like Momo, so it will be interesting to see how she deals.