So this is more of a niche hidden feature but I suppose I can reveal it , There are 6 straight up corruption girls that don't have redeemable paths or anything but are there for your several events or corrupting yourself further too.
There are girls that you can either corrupt or help to get their life a little bit simpler(both the paths get you sex in the end but one obviously is more on the side of devotion and harder ) , the corruption path is you corrupting them slowly and the help path is you helping them which turns their purple hearts into red ones if you take the correct decisions.
Then there are the red hearts , who will either be a part of the main story exclusively or will be your alternate romance endings down the line.
Now while we have the heart system obvious and clear but for the ones you can convert you gotta figure those out by playing and paying attention to how they are personality wise.
I guess the most non spoiler of this would be Zoey who is one of them , rest hope you can figure out