I guess this was done on purpose, because if Rachel was a good person we'd feel bad for her. Maybe the author is worried that if Rachel turned out to be a genuinely nice girl and decent ruler we would root for her instead of Marie? I don't feel like Marie wants to be part of the nobility anymore much less royalty so if Rachel was a good candidate I would've been upset if Marie still ended up as empress.
Honestly I think having supportive female characters towards FL is the best way to make us feel good about the story. But just like you said stories that only feel good are boring! The bad things that happen in a story make the good moments even more sweet and precious!! It's like eating icecream after swallowing some nasty medication!
Also, I can't feel good knowing Marie is going to choose to become empress just to be with the one she loves. I kind of want a story where the ML chooses to leave everything behind and start anew with the FL. Why must it always be only the main characters that are fit for leadership?!
I get it but also like its so boring to have just one good female character. Like this is a trend where always there are abundance of perfect guys (hence strong 2nd male lead syndromes) but rarely do we ever get a likeable female 2nd lead that's competing with the female lead. As if all women can do is be a bitch to each other cause of love. This would've beed a good opportunity for the author to show us that while there are still other good choices to be the crown princess, Marie still would've been the perfect choice. And it can give character depth too to the other girls instead if just another run of the mill rival. When writers just give us another bitch of a female rival, it always feels like the author is leaving the readers no choice but to root for the protagonist because what choice do we have if everyone else is horrible and selfish and one dimensional. Plus imagine the development/drama the main character would have too in order to show us that she really is the rightful lead of the story.
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Rachel, she's using Marie as a tool to become a crown princess and
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She's a b***h