I'm pretty sure something happened with Kanata's engagement, which is why he warned MC. Honestly, I think he's MC's best friend. MC may be good at charming girls, but his biggest weakness is that he's completely clueless when it comes to their feelings. It got so bad that Rei started giving off yandere vibes

and I'm calling it now, the girl on the last page is probably Rei's younger sister.
thank you for translation & update again after 2 months