Really? I felt it was opposite. The story for a long time makes it unclear what Yuma's feelings are exactly and Yuma doesn't know either and goes from being confused and not even realizing that it is infidelity, then realizing it is, then being confused about what to think about Hotaru, then going through a phase of thinking it doesn't matter what their feelings are and what their relationship so long as they're happy and then finally being pretty clear and explicit about being in love with Hotaru and having been for a long time whereas what Hotaru's feelings are remains ambiguous till the end. Could have been in love with Yuma all this time for longer than Yuma has, could've also just been been clinging to a friend but even in the extra chapter it's still not really clear what their relationship is no. Hotaru is just “You're the only person I really care about and I don't need any other friends.” and that's it.
Actually, before this Naoko Kodama also released a shorter story that eerily looks like the same thing but from Hotaru's perspective as in the protagonist who is someone with really only one friend while that friend has many friends, but that friend is in love with the protagonist and the protagonist knows that, and uses those feelings to tie that friend down, while also not really having any real desire to date that friend so it really makes one wonder whether that was the intent behind it. Naoko Kodama does this kind of stuff. Most artists just make “They fall in love now they're going out.” stories but Naoko likes these more asymmetric, more complex relationships.
I'm thrilled they are together, however, Yuma doesn't see black/purple hair girl as more than friends bruh. Their relationship is more like friends w/ benefits, if u ask me.