Looking for clues that might give away Ririha’s reasons on why she acts the way she does, I focused on her comment “Nothing but idiots” (ch20). One of her friends says that 4th/5th grades boys are all idiots, and Ririha darkens at that, as if bearing a grudge against them. Besides, the fact that she attacks Yuzumori and tells her those words, referring to something very specific, makes me believe that she’s driven by a kind of resentment that can only be caused by a broken heart, or pride. Yuzumori is oblivious to what she’s talking about, so I assume that Ririha was in love with a boy, the boy probably didn’t find her pretty enough and had a crush for the prettiest of the class. Yuzumori probably wasn’t into him, but it was enough for Ririha to decide to work on her figure to the point that her face would appear on a fashion magazine (or maybe she was already a model, an affront that hurt her pride twice as much). When the chance to return the favor to Yuzumori presented itself, she couldn’t just let it get away.
But, it’s just a supposition.