There's plenty of ways Ririha can "rob" Yuzumori from Mimika.
Just denouncing her as a pervert lolicon should be enough to make their relationship "complicated".
I don't see why that would work. She's a little kid making allegations about a girl, in a sexist society. Sexism cuts different ways--people aren't going to be disposed to see a woman, let alone a high school girl, as a sexual aggressor. And they're not going to be disposed to pay attention to the hysterical irresponsible claims of a little kid--maybe if her allegations fit a classic frame, like if she was accusing a male stranger, and if the victim backed her up, but only then. In a case like this? Likely they wouldn't even give her the respect of politely doubting her, they'd just scold her for being a troublemaker/having an overactive imagination. And when Yuzumori-san calmly and maturely ripped her claims to shreds? Fuggedaboutit.
So yeah, I don't buy her ability to successfully blackmail; she's got nothing on them. I don't even see how she has a basis to suspect what's going on. It's not like we have any indication Yuzumori-san has a history of lesbianism (which would be kind of bizarre at her age). All she can plausibly know is that Yuzumori-san has a friend or something who she seems happy to be with. Of course, she could lie and have her lie accidentally happen to be the truth--but as I said, I don't see why she would be believed.
