Responses
I think, the backstory is necessary because it tells us of why he doesn't talking to other people. He was betrayed in the past so he avoid socializing or talking personally or having any kind of relationship to other people in the present.
I also think that he didn't revenge to his ex because I think for few people who experience trauma or have a bad experience in the past choose to avoid any contact that is related to their trauma. So I think he just doesn't want to encounter again his ex who betrayed him.
After reading so many revenge stories i feel like i got blue balled. It was such an interesting backstory and potential plotline but they do nothing with it; very unsatisfying. What was the point of adding that detail besides pity points? It honestly just soured the romance.
For example, when Kiraide Isasete revealed that he was r*ped in the first volume they resolved it in the secondth volume. Really tickled the good spots.