One topic that I find interesting to explore is Rumi's childhood, especifically the parts where that are relate with Rumi living experiencies of a normal child (and how that may have also influenced how she sees herself in the present )
What was Rumi like at school? Did she has a lot of friends? Or just a small group? Did they make comments about her hair? Rumi grew up in the early 2000s so I assume that the rules about hair/appareance at Jeju/Seul schools were much stricter compared to today. Celine had to search through many schools to find one that did not make Rumi to dye her hair. (Or maybe Rumi had to dye her hair and the Idols Awards photos just was when she was in vacation. Ooor Rumi has allergic to dye and the enrollment meetings were a nighmare for Celine "Yes, it's her natural color. No Rumi cannot dyed her hair. Here's the doctor's certificate. What do you MEAN—" Yeah. these aren't the most cherished memories for Celine and Rumi.)
What if Rumi (around 6 or 7 years old) showed her patterns to one of her friends?. ("Why would she do that?'' You wonder) Well. Maybe Rumi thought that Celine was exageratting , one of her classmates had a birthmark on their neck and it was no big deal. Yeah, some classmates were mean to them, but they were horrible since the beggining of the school, so she went and showed them her patterns a her best friend. Hae-Won just make a few curious questions just to end saying that it looked pretty. And then everything continue normal.
Everything was normal until Hae-Won rambling about their day with their parents mentioned the marks on Rumi's arm, which, according to Rumi (who was repeating her doctor's words), were due to a skin condition. And her parents not knowing nothing about Rumi skin's condition, feared it was contagious, told their child to stay away from her. And Rumi end up losing a friend with the only context that the parents of their friend don't want her because her patterns .
(If you want a more angst scenario, Hae-won's parents go to talk with the principal to express their concern for their kid safety, and perhaps they also share this concern with other parents. This leads to everyone in the class learning that Rumi has a little-known skin condition. Rumi doesn't end up being ostracized, but she finds it very difficult to make friends from that point on. Perhaps she eventually becomes friends with the child with birth mark and their friends.
(What if that friendship had lasted much longer. Maybe until Rumi began her full training as a Hunter and Idol. I would be funny that in some point, at one of their reunions her friend arrived with the news "My parent got the money for the surgery "
Friend: Yeah for get rid of my birthmarks :D
Rumi have complicated feelings about that )
Celine's reaction in this scenario would be more fear/worry than anger (Did Rumi show her patterns? What if a demon knows?! What if a demon already knows?! What if a demon tries to kidnap her when she's not around?!) This leads to her, while not scolding Rumi, raising her voice considerably as she expresses her concerns and explains the potential danger. And Rumi is seven and knows that did something she shouldn't have and the things in the school are a little bit bad and Celine is talking very loud and …and… *cries* (And maybe after calm down Rumi and making sure she is asleep. Celine allows herself to cry alone in her room.)
Another thing to explore are doctor checkups. Rumi's doctor is probably one of the few people who know her patterns, of course, they know nothing about demons. Rumi simply has a unique skin condition. The same goes for her hair. I have this thought, that Rumi's purple hair comes from a molecule unique to her, called "Rumelanim" in the Kpdh world (And her skin codition end up called Rumiatus pilaris)
So Celine and Rumi has her medical records to back her up when people ask about her hair.
Miyeong's reputation is interesting point too. Celine could talk about all of Miyeong's good qualities . But Miyeong was an Idol who had a baby outside of marriage a late's 90. The only reason most people don't speak ill of her is because she died. But I said most, not all, for a reason. Since Rumi has been in the music and idol world since she was a child, she's likely heard some rather… colorful comments about Miyeong. And those comments were going to affect Rumi no matter how much Celine reminded her that Miyeong was a wonderful person and that the people who said such things were awful in every way.
But Rumi watches as Celine works herself to exhaustion for her, as Celine is trying to fullfill various roles alone (and Celine should not be alone) the best she can. So some nights, after help Celine with her wounds , when she cannot sleep because she fears Celine might stop breathing at any moment, when all she wants is for Celine's life to be easier,. her mind replays everything she's heard, and all she can think is… What if…?
Another point to explore is Rumi feelings about she shares a trait with the same creatures that:
a. Are killing people
b. Were hurting her Guardian Mother and much later hurt her friends
And how that fact affects her mentally. I remember read fanfics that addressed this topic but it's most were from the perspective of "Someone said demons are horrible creatures" and Rumi go "This totally about me because I am demon"
And i think that Rumi should have a most active role in that thought that just… be affected by every bad thing that someone said about demons. Rumi witnesses daily how demons harm innocent people and her family. I think she deserves to have her own thoughs of the matter and not be only a eco chamber of "I am horrible creature because everybody is saying that demons are horrible creature"
The scene in the roof where she says "Not demon, just a hunter" it gives up a glimse of how Rumi (and by extension Celine) were adressing this topic. I feel that Celine make the mind olympics for have this information → "Demons have patterns" + "Rumi have pattens" and go : "Rumi is a hunter and she have patterns and she's not a demon". And she seek to convey that logic to Rumi but Rumi just heard that and went: "Mmmmm.. This don't make sense"
Celine: No thought it to much (I don't want to think in the implications of the other possibilities. And what that said about me as a hunter)
Rumi: Oh! … Okay (This don't make sense, but if Celine said it must be true. BUT This don't make sense, BUT If Celine said it must be true but…if Celine said it must be true, if Celine said it must be true, if Celine said it must be true )
And when the the events of the movie unfold. Her patterns are spreading , her voice is damaging the honmoon and the "I'm not a demon'' logic were crumbling to her (which is why, as the film progresses, she becomes very affected by the demon comments but at the begin of it she participated in the demons bashing). And that's when the logic of consolation emerged: "Jinu isn't completely evil" + "Some demons can be good" = "If I can prove that demons aren't completely evil, it means I can be good."
(Idk, I feel that the popularity of Rumi is homeschooled headcanon not have allowed the exploration the others aspects of her life that might have contributed to the "I am a mistake" thought. I have nothing againts that headcanon, it just that, in most cases, the narrative use it to emphasize : 1) Rumi's ignorance of many common facts (which are greatly exaggerated most of the time). 2) Made every little thing that is wrong with rumi perception of herself Celine's fault. (It feels as if Celine, like clockwork, went to Rumi's room and said, "Rumi, I hope you don't forget that you are a mistake. Do you remember that? Let me repeat it. Better yet, I'll spell it out.")
I feel that the thought "I am a mistake" is 100% her own conclusion about herself, It's not the result of something Celine said to her directly, I think the most accurate way to put it is that Celine, unconsciously, prepared the ground for that thought to flourish, but never planted the seed. (Or perhaps a better metaphor would be to say that Celine planted the seed, but never nurtured it. Idk, Choose your metaphor) In short, the birth and consolidation of that thought was simply (insert everything I said in this post).