@Starbright I think you need to re-read the last chapter. How was that a sad face? It was a happy face where he's glad that she's okay and things are back to normal for her in a friendly manner, not neccessarily romantic. Ofc though, he does have attraction for her(as we saw when she was in volleyball uniform) but in terms of romantic feelings we are only shown he despises her actions. It's quite possible he has some romantic feelings but as he seems to be a very straightforward normal thot-slayer, it's unlikely at the given moment.
But ye, in reference to you calling ntr on this. It's fine to have your own fantasies and you can like what you like but this is a romantic comedy. There's plenty of series like this with the main duo getting into similar antics and there was never NTR or whatever. Just relax and enjoy the series for what it is. Or go read a hentai if you want some NTR.