For all the people hating on Dan-I, I would like to redirect you to chapter 30, where Chunyoung states that if he were dying, he wouldn't tell his lover and would just break up with and leave her. Why? So that she wouldn't feel as much pain. If you didn't hate on Chunyoung then, you're a hyprocrite if you hate on Dan-I now. This is the EXACT same thing, but 100x worse. If one day Dan-I permanently returns to her world, the others will forget about her. Rather than telling them about this and having their time be stained by stress and worry and suffering, she prefers to keep t o keep this misery to herself and enjoy her moments with them. This is besides the fact that telling them they are fictional characters would likely shatter their minds and give them a huge existential crisis. Perhaps you would be different from Dan-I in that situation, but can you not understand how she thinks at all? This isn't "misunderstandings created from lack of communication," it's "weighing the pros and cons of telling them and since in Dan-I's opinion the cons far outweight the pros, she decides to not tell them."
As for Dan-I thinking Ru-Da is a female, has Ru-da ever acted differently in front of her??? Would you guys know Ru-Da's gender for a fact IF YOU DIDN'T READ THE SPOILERS?????? Ru-Da being good at fighting? There are girls good at that, too. Ru-da going into the mens' bathroom? In Dan-I's viewpoint, Ru-Da has to keep up pretenses. Ru-da has a flat chest??? Maybe she has a flat chest or is binding her chest. There are so many tropes in this manhwa that parody web novels tropes (just look at the reasoning of why the fighting ranks had to hurry and leave a few chapters back) . Can you honestly blame Dan-I for thinking of a trope that fits right in?
As for Dan-I not knowing that Ru-Da is two-faced and can be incredibly manipulative, Ru-Da has never shown that to her in any great extent. The main two points were 1. with Yeoryeong and 2. when Ru-Da asked her to be friends. In both cases, Ru-Da didn't show any bad intentions. I mean, he doesn't want to hurt Dan-I in the first place (not sure if he realizes that breaking her and the 4 Heavenely Kings + Yeoryeong would hurt her, since from all Ru-Da knows, she's been friends with them for 3 years yet can't trust them, has self-confidence issues, seems to be more submissive in the relationships (aggresive phone calls), etc). Rather, he has done a lot for Dan-I. This is added to the fact that Ru-Da is the only person Dan-I has ever opened up to. Ever. Remember that the last time Dan-I tried to be friends with someone outside of the 4 Heavenly Kings and Yeoryeong, the 'friend' turned out to be a b*tch. Perhaps Dan-I has an inkling that Ru-Da is a bad person because of subtle hints, or perhaps she didn't. Either way, of course she doesn't want to believe that the one person she ever opened up to is bad person, nor is she going to think of that person being a bad person at all unless there was a reason to.
Dan-I has been alone. Sure she has friends, but there is also the knowledge that she is 'real' and they are 'fictional' that separates them. If you've never had the knowledge that a relationship will end but decided to continue with it and happily live in the moments with the other person despite that knowledge, then I envy you. Because it's a fragile happiness. I'm really tired of all the comments bashing Dan-I and calling her an idiot because trust me, I have seen idiots IRL and read stories with idiots and she is not one. She is relatable. The most relatable fictional character I've ever seen, to be honest. She has insecurities, she's never been noticed, she tries hard for her grades but her parents never tell her so much as a 'good job.' But she also has her strong points, she can stand up to herself when needed, etc. She is a human, and just because she's surrounded by people who seem perfect doesn't mean you should expect her to be the same.