I read your reply to the anonymous person who talked about Kill Me Now, and I'd like to suggest a scenario that has crossed my mind
Miyeong lives and raises Rumi to be a hunter (bonus if she's with Third and Demon Father), but Celine has completely disappeared and there's been no news of her for over two decades.
During a world tour with a stop in France, at some party or event with drinks, Rumi and Celine meet, but they both give false names when introducing themselves.
They have sex that night and agree to see each other again in two days. Rumi says she has to leave, but gives a private phone number. They don't talk every day at first, but then they start talking more frequently (due to some event in the plot).
Rumi starts learning French, and the others are confused as to why, but let it go (except Miyeong and Third, because they suffer remembering Celine).
Time passes, and they notice that Rumi is more scattered, and the notoriously workaholic Rumi is now scheduling all her days off and researching flights (Celine's new job requires her to travel, so Rumi and Celine are having dates around the world), and this is VERY STRANGE, especially considering it's Rumi.
Celine finally comes to Korea, and the very next morning, when Miyeong, fed up with Rumi disappearing, goes after her. And there are Rumi and Celine being interrupted, and then the discovery of their true names and identities
(Miyeong is jealous of her own daughter because she loved Celine romantically, but never told her KKKKKKKKKKKKK)
ok this skirts the line in a way I like. From Rumi and Celine's perspective its completely normal. But from Mi-Yeong's perspective its insane.
I have a lot of things to say.
Rumi's workaholic nature comes from Celine in canon. she works so hard because she wants to erase her patterns, without that pressure from Celine that her patterns are a problem I don't think she would be workaholic, at least not as much. Unless she also gets told that by Mi-Yeong and thirdlight. considering Mi-Yeong banged a demon (I think consensually) I don't think she would push Rumi to do that. But if Rumi was not consensually conceived I think she might pass down her hatred for that demon to Rumi thus making her a workaholic again.
This gets more interesting if Rumi a half demon. Celine has a high chance of leaving if Rumi is a half demon. And if Rumi is still hiding that fact (thought I doubt she would if here mother was there). but for angst sake lets say she is. I can already imagine the scene of her finding out. Huntrix tower is empty as Mira and Zoey are in america visiting Zoey's family. Rumi invites Celine over. Rumi reveals her patterns to Celine and it does not go well. Celine ends it right there and is storming out of penthouse when the elevator doors open and Mi-Yeong is standing there. Celine puts two and two together and realizes who Rumi is and how much she fucked up. Mi-Yeong is stunned she hasn't seen Celine in 25 years and is so confused on why she is here. They don't say anything and Celine leaves. Mi-Yeong finds Rumi in tears in her room. God it would be so bad but so good at the same time.
there are three ways I see this resolving. the first and most boring is Celine just leaving again and not talking to anyone. the second way is Rumi and Celine make up an get back together. the third is Celine apologizes but then gets with thirdlight/Mi-Yeong and Rumi gets with Polytrix. I last option works best if Rumiline wasn't a long term thing and wasn't that romantic
This is one messy AU and I like it.