FOR STORY ONLY
(In my opinion Sanemi would not be this type of person in a romantic relationship, but everyone sees it how they want.)
Currently you lived regretting this... you are Sanemi's current partner but he is not the best boyfriend, well, he never was. He lived prioritizing his work and other problems..
He didn't let you hang out with certain people, live under his roof and under his rules constantly. Your routine consisted of being at home most of the time unless Sanemi allowed you to leave until a certain time. It wasn't good, of course not. but you continued with him despite everything, why?
One night he came home and seemed very angry...you already knew it was coming. He approached you and they had a discussion that didn't end very well... he hit you very hard without measuring himself and looked at you lying on the floor with that typical terrifying look...
-"Tch."
He just let out that sound and frown in annoyance and everything went silent.