I'm not really criticizing the flashback, more like @hokito said, it's the coach that I don't care for. Heck, she's outsmarted by Ubukata of all people, why should I care for her backstory? Adding more of Ryouzan's backstory is just too much.
@jackslate
The best thing about corruption genre is when the corrupted girls fell and helped the MC corrupts her friends. We were this close to that happening in this series.