Hikari is totally becoming a decoy protagonist at this rate.
inb4 the first step Ayami does is to break up with him for her "revenge" at life. We already know from the present that she considers Yuu her ex-boyfriend, so at some point this (technically) vanilla experience for her and Yuu (not for Hikari, OFC) is going to end before the present day where Hikari catches them in the act.
I'd love to blame Yuu for not telling Hikari he was in a relationship before, but the key term here is "
was" since they've already broken up when Hikari started making moves on him. He was probably feeling very conflicted when on one hand, his old crush is sending mixed signals probably starting to make moves on him, but on the other hand, there's his ex-gf who he can still get back together with.
Anyway, I was wondering what the previous shitstorm was about, but now that I caught up it's BSS instead of NTR (still bad mind you, but at least Hikari can cope that Yuu was never hers, so he technically couldn't be stolen from her). Now, if Yuu and Ayami officially rekindle their relationship, then this technically turns Hikari into the homewrecker if despite that, she still decides to pursue her feelings for him.
inb4 author wanted to write a story where the readers were rooting for the homewrecker all along
EDIT:
By "revenge", Ayami probably meant
リベンジ (revenge, second meaning) which when used by Japanese typically means to take another shot at something. She wants to have a second go at happiness in life with him in spite of her shitty circumstances. So now that makes me more curious about why they even broke up in the first place.