At this point, it's safe to assume that most, if not all, of the characters are connected to one another in some way beyond just being classmates or related to one of the girls in the classroom. There are connections that aren't immediately apparent, like how Ami, Akira, Miku, and Kaori's lives were all disrupted heavily by vehicle collisions that may or may not be the same accident, or how Akira's brother may be Sana's father.
Lately, we've seen conjecture that he might also be Chika's father.....but who else? We have a lot of girls who seem disconnected, how do they tie in?
Bold Theory That Might Be Full of Shit(tm) #1 -- Maki and Nozomu were lovers. She was in an open marriage, he fucked around, and there's a massive amount of tragic irony in Makoto's mental health being invested in "making Daddy proud of me" when the man she was trying to please isn't actually her father. Liklihood of actuality: 3/10. One would think Maki would mention that she had fooled around in the past with an Arikawa Teacher in the past.
Bold Theory That Might Be Full of Shit(tm) #2 -- Nozomu was Rin's sperm donor. There had to be one, right? Actually, I can't recall if Rin is fully adopted or was donor-conceived, but either way, there's a mystery dad. Liklihood of actuality: 5/10. There's no reason to believe this is true, but there is exactly as much evidence to believe that it is.
Bold Theory that Might Be Full of Shit(tm)# 3 -- Chinami's benefactor -- the wealthier woman who looks nothing at all like anyone in Chika's family who took Chinami to her doctor appointments was.....either the mother of another one of the girls, or one of the adult women in the game. I'm not sure who. We know it wasn't Tsubasa, as she would have said something during her story. Could it have been a pre-addiction Yuki? I'd think that'd be brought up. Sekai? Very possible -- she looks nothing at all like Chika's family. Likelihood of Actuality: 7/10 -- it feels like too big of a reveal to siimply leave as "mystery woman," but I think we'll eventually get more information about this down the line.
More as they poop into my brain.
That was supposed to be "pop," but I'm leaving the typo because these are probably all shit.