from what i understood, sachiko killed someone probably his mother coz she mentioned wanting to become his real mother or it could be his father too seeing as we see no mention of him after the childhood in the chapters.She probably forced or atleast had sexual relationship with haruhiko kanna since in pg 18 vol 2 ch13 she was half undressed and climbing into his bed.also in the last chapter in the pg 10 she was clearly talking about doing it with him.
I understood it more as her planning to seduce the father in order to become Haruhiko's mother.
Haruhiko's real mother was at the end of her life at the hospital, so maybe the father fell for Sachiko's seduction.
If Sachiko did indeed kill the mother or father, the characters wouldn't be fine with her being out and about instead of behind bars, and Haruhiko would have no sympathy for her.
can anyone tell me what happened with sachika in his childhood.seems she went to jail for it and all but its not completely clear what happened atleast to me.can someone please tell^^.thanks