No promise was made, it was only in her mind that it had been set. Perhaps she thought she was secure because of that but all it lead to was abeyancy. Had she been more forthright in her desires perhaps she would have gotten Oh-chan.
Dude gives off NTR antagonist vibes lol but he doesn't really do anything too bad. This is probably the closest the "let me train you to be sexier to appeal to the other guy" spiel ever came to making sense because Oh-chan really DID go for the other girl entirely because of sex appeal.
This actually turned out to be a good couple of chapters. I’m glad author decided to take her down the route of finding someone new and moving on rather than make her a yandere instead
Damn it Oh-Chan why u had to make her sad! Glad we got a sequel, I had hoped she would win over Oh-chan back but unfortunately that bad senpai sunk his claws on her first!
How is he a bad senpai? Literally never forced her or blackmailed her to do anything. Just told her the truth and went from there. She was vulnerable and made the choice to do it with him of her free will. And ya know i hope both of them are happy with the new love they found. Not all childhood love stories have to end with them getting together for a good ending.
He did take advantage of her being vulnerable which is definitely not the best morally, but for a doujin he was actually really respectful of her boundaries. He didn't blackmail her, threaten her, or force himself on her. He even asked for consent.