I seriously hate this lame angst trope that all manga seemed to feel obligated to go through... I've seen it sooo many bloody times (in the more than a decade I've been reading manga), and it's never compelling- just frustrating!
Honestly, this is why "Kase-san" is so good, because it doesn't seem to have any obligation to adhere to hackneyed cliches that get in the way of interesting story telling. I almost have to wonder if the mangaka was being pressured by their editor to insert some kind of 'twist' into the plot? Ririha suddenly comes out of nowhere, with no clear motive, and is way more vindictive and cruel than any person her age should be capable of being.
One of the earlier commenters summed things up pretty well: The manga starts out with an extremely entertaining concept, then devolves into something that could be summed up on TVtropes.
As and addendum, after thinking about this... I think the what's going to irritate me the most is that once the period of perfunctory estrangement/longing ends, the story will 'resolve' things in such a way that the whole damned thing might as well never have happened... sigh. Which would be both a relief and a frustration, for reasons I frankly don't have the energy to get into at the moment.
Honestly, I might be better to blow this manga for about 6 months, then come back... once the whole ordeal has resolved itself and moved on. Right now, I'm still inclined to give Ejima more credit than the bastard making Citrus.
The worst thing about these kind of tropes is that they almost never serve a narrative purpose beyond just extending the length of the story. It's almost never used to advance the character arc of either of the characters involved it's just padding until they clear up the misunderstanding and then proceed to act like this extremely presumptuous disregard for the other person's right to self-determinate never happened and doesn't need to be meaningfully addressed.