I think that Rei is just a fragile young girl with a confused heart. Takashi is the one who failed to return her feelings, making her turn to Hisashi. You need to peruse the first act to get the background story.
As for Saeko, I don't know. I've never been attracted to expressionless killers, so she is at the bottom of my high-school-girls list. Incidentally, she is in the same position when it comes to Komuro's high-school-aged love interests.
Alice is pretty cute & acts her age without being a smartass, which I like.
Takagi would have been a strong competitor to Rei for me if it weren't for her wearing glasses. A Love Triangle with two tsundere is always interesting (& rare), though.