I know this is focusing on Rio and the reasons she ended up where she did in the events of the main story.
But as two-faced and miserably jealous and damaged and awful as Rio is, as abusive and sociopath-adjacent as Itsuki is, and as futilely jealous and depressed as Kyouko was,
I cannot stand Rei and I truly think he was the worst of the bunch. Rio made terrible choices and shouldn't be forgiven for them, but I at least see the through-line of how she got from Point A to Point B, as dumb as it all really was.
Rei is .... just an asshole, thinks only with his dick, is lazy and conceited and egotistical, and I really can't wait for the Special Bonus Chapter that shows how Mahoro ended up with him.
I'm sure it's something as simple as "he was handsome and charming and she fell for him like all those stories", only to find out (too late) that there was nothing worthy of her behind his smile.
But Rei is the one that I'm happiest about "losing" at the end of the main series. Even more than his mother - she might have raised him and instilled the value(less) system he has, but he's still an adult who's responsible for his own choices and actions, and other children have broken away from bad parents before.
He has no excuses.
But, yes, Rio clearly "pushed herself onto" Rei, contrary to what she claimed at the anniversary party. I figured that was the case, but seeing it spelled out is still illuminating.