I think you are mistaken about something. Most of us are not mad at the fact that they ended up together. We are mad at the way the author made it happen. This love story have absolutly no basis in the first timeline if not physical attraction or Vita forcing Tia's feelings to keep her in her place. She was brutally abused, ended up dying and got a second chance by pure chance (because time was forced back for Ji-eun sake, not Tia's sake). Then we get to the second timeline were Ruve STILL act like an asshole, neglect her after putting her into a comatos state and Tia get debilitating PTSD.
And what happen next? She quickly get over it enough to start pinning for him again (toward their fifth meeting I believe) when he did nothing at all. We can only conclude that she is still in love with him despite everything. And that does not make any sense : attraction is not enough to overwrite the abuse and she literally got a blessed name making her able to go against her "fate" (AKA the crown prince ending). Seriously, they don't know each others at all by this point and she should only have negatives emotions toward him... But no. Allen and Carsein were the one who were always on her side and helped her get better and enjoy life again. Ruve was going to be an ass because of his own victim complex, jalousy and prejudice toward her. The lies only worked because he was ready to hate on her and the poison did not take away his rationality (if not he would habe abused others too). So, the ML is a past abuser with an horrible and overbearing personnality who get everything ended up to him. That is not even taking into account how the author is low-key victim-blaming Tia for what happened in the first timeline (even more obvious in the novel). It is a mockery of trauma and a very disturbing storytellling. The fact that he is not First Ruve does not erase what happened to Tia, because she remember everything. And she is the one doing all the hard work again? Right...
How people can be so rude about disliking Ruve even now? I don't understand any of you.
Carsein is good, he is a kind and a sweet boy. But it's just obvious that FL will not be with him and choose Ruve because the aura of those two has been like that.
He should better be with a sweet girl who loves him and be in a reciprocate relationship. All of those characters needs to find happiness and Ruve's happiness can't be without FL, as well as FL's. So if we want happiness for Carsein, we should just accept this reality and continue with the flaw.
It's my point of view. I like how Ruve isn't perfect. He did bad things in the past but didn't in the actual reality. Aristia was also imperfect. She said it too. She used to be cold with anyone and only think about political affairs. They both changed for the better. So why not agree that they can be together?