He pretty much stalks the short hair girl all his life. He's in love with her but she has no feelings whatsoever about him. It's an entirely one sided thing and has been for years which alone is pretty weird.
Em, stalks? He just watched her from afar. Honestly, I was like him with my first love when I was 11-14 years old. When you love someone at this age then you enjoy even just watching them or thinking about them, but you are too afraid of rejection to even try and be friends. I think this novel portrayed a young guy who is in love very well. Also, it explained why they "hated" him - it was in a dialogue with his sole friend. They just enjoyed watching him being "secretly" in love with a girl who is everyone's slut. It makes sense for her to hate him because they are completely opposite - she is a dirty slut, so receiving his innocent love could make her feel what piece of shit she really is.
Also, it seems people don't understand the definition of NTR. The only thing that matters is feelings MC have for the girl, it doesn't matter if she loves him back or not. It could be one-sided love, it could be mother, close friend or sister - there all sorts of NTR. This is definitely can be counted NTR, the only thin that lacking is build-up. The author should have added some flashbacks from the past when MC first started to like the girl.