^Now that you mentioned again about the potential person Senpai's unrequited love for, I wanna share my first thought when I read those part.
Maybe I was biased cus this is GL lol, but I really thought that the person Kisaki has a crush on is also a woman, cus maybe that's why Fuyuki and her got close. At first I thought she had her own childhood friend as well but in ch.7 she said "Being CF is pretty troublesome, huh?" the way she said that, I felt that was unlikely.
So, the next thing I thought after reading ch.9 is maybe her crush is a woman who lived in her neighborhood? Like a really close friend that have been together not long enough to be called CF but long and close enough to got treated like their own daughter even by her husband. I just thought those panel showed how disgusted she was to be treated like a daughter by the love rival.
When Fuyuki asked 'is that why you left home?' and Kisaki replied 'I didn't get along with my mother either, to begin with,' and went on to say 'the trigger was when the two got married...' I just thought like that was two separate things. Like I dunno I never thought about the possibility of her crush to be her stepdad before reading swossy's thoughts on here. Well even though I really don't want that to be her stepdad, I've to admit that it does makes sense when you read those part. But you know, I still wanna hope that those part is just misleading by Shinoa sensei lol