After some thoughts, I now realize what is bugging me with Riko's character. Its the discrepancy with her being super beautiful AND being hallow/sociopathic(?) at the same time, but still being the most popular girl.
It is like you play the most photorealistic PC game ever, e.g. like Sword Art Online realism (or whatever), but the rest just su*x a*s. Controls are on ET (80s) game level, the story is super boring and there is nothing to do. What I mean is, there must be something besides looks to be attracted to someone , so you want to keep playing/spending time together. What happend to Sae must have happend to the other friends as well, e.g. they told Riko about something that happened to them, but Riko skipped over it, ignored them and continued on elswhere. This would hit everyone and you would second guess to spend time with someone like that just to be in the "popular group". Maybe its an other thing, because Riko is just part of the group and basically something like a mascot, that rises the appeal of the group as a whole (probably bad English here ^^), so that the conversations with all the other members of the group are interesting enough to keep someone like Riko in it to raise their status.
Sae didnt have these options. She was alone the whole time and when she got someone who cared for her, he was stolen from her. So yeah, writing this I came to a conclusion, why Riko is being kept in the group, but she probably doesnt have any friend at all.