A theory about Rumi's patterns
This one is a little random, but basically, we know that patterns form through shame, or at the very least that shame is a major part of the process, right? And it seems unlikely that Rumi had her patterns right from birth because babies don't feel shame. This is something that would also fit with Celine's comment of not imagining a child like her, which is something I've seen other people theorise. There's also that picture from the scene the fans are talking about Huntrix, where little Rumi is standing with Celine at the Internation Idol Awards as a child wearing a sleeveless dress. Yet, during the flashback scene at Miyeong's grave, she is shown to have some patterns, just a few though, suggesting that it's at that point in her childhood where the patterns first started.
Anyway, I think the general assumption from there is that this was when Rumi first started feeling shame over her background. And yes, the shame she developed over the years is very specifically about her father and her demon side. The shame of Rumi's that propels the movie is about her demon side. But what if that wasn't the shame that started the patterns growing?
What if it was shame about her mother?
I'm not talking about the Hunter side of her. No, I think she'd have been raised with pride about that right from the start. I'm talking about the rest of the story. That she had a child outside of marriage, that the father was not in the picture. I mean, he could have been in the picture, but since we literally know nothing about him the chances are the public didn't know his identity or what he looked like or anything. And while in the film's universe fans are shown to be kinder than some IRL K-pop fans, this was still twenty-something years ago. So attitudes/standards around idol's personal lives would definitely have been less merciful. And the fact that Miyeong not only apparently had a secret relationship, but one that resulted in a child? A bastard child, no less? Chances are, it would have been considered a scandal even in this universe.
Because Celine started raising her, and training her to follow in her mother's footsteps (in both Hunter and idol senses, though the public only know the latter), this may have ameliorated some of it, and she may well have capitalised on any goodwill towards The Sunlight Sisters as a whole to help. That, and the fact that Miyeong died, turning scandal into tragedy, and so something that the fans could show sympathy rather than outrage over (I actually think I may have seen a post or fic somewhere also making this specific point, actually). Nonetheless, it would still be a bit of a cloud over everything.
For Rumi's first few years, she'd be too young to understand that. Celine would simply explain Rumi's mother died and couldn't look after her, and would gloss over the matter of her father completely. She'd try to shield Rumi from any horrible comments too. But as Rumi gets a little older, no doubt she'd wonder about that. And of course, she would start going to events with Celine and mixing with other idols and celebs who knew Miyeong too.
So maybe, at the particular Idol Awards where she wore that dress, she got talking to some of these people. And maybe, while Celine was busy networking with someone else, she asked about her mother. Perhaps one of the people she was talking to was actually madly jealous of Celine/The Sunlight Sisters. Perhaps they just didn't like children and wanted to upset her so she'd go away. Maybe they were just thoughtless, or drunk, or whatever.
Whatever the reason, someone tells her her existence was a scandal. Likely, someone else drags the instigator away because, hello, this is a literal child you're talking to here. But not before the damage is done. And then maybe on the way home, Rumi asks Celine if it's true her mother wasn't married, and she says yes. Maybe, because I imagine she's a pretty perceptive child, she asks if the only reason people aren't more mad about it is because her mother died. Celine wouldn't agree with that to Rumi's face, but maybe something in her face and hesitation before answering confirms this to Rumi. Anyway, it makes Rumi think more about her origins, and the shame behind them. She starts to feel the shame herself.
And then the patterns show up.
Just a few, just a little. But Celine, of course, freaks out. She never wanted to believe Rumi could have inherited anything from her father, tried to deny it. If she could have gone Rumi's entire life without telling her that her father was a demon, she would have. But now there's no choice, so she has to explain it. And she explains it needs to be covered at all costs, that they must never be seen, just like faults and fears. That it's a problem to be fixed, and that it's okay, because what's more important is that Rumi is a Hunter, like Miyeong was. That as a Hunter, she can fix this. The flashback conversation by Miyeong's grave that we see in the movie would therefore be a part of this process.
So her shame 'switched sides' essentially. Instead of feeling ashamed about her mother, she started to feel ashamed about her father. And that shame grew and grew until we get to the point we see her at in the movie. But it would still be the same shame.
