Ok i personally think this is a fluffy-ish outcome to an angsty situation but i'm well aware my angst-o-meter has been broken for years so. Here's another idea for gumiho celine au.
Demons are, canonically, shapeshifters.
What if Celine subconsciously kept checking baby Rumi (and then kid Rumi, teen Rumi, adult Rumi) over for gumiho characteristics, and Rumi kept subconsciously adding those to her understanding of 'what -i- might look like as a demon', to the point that, as an adult that finally lets her demon side out, she ends up looking not only more gumiho-like than she should as the kid of a jeonseong saja, but also she looks kinda like Celine's fox family line.
Like. Sajas do not have pointy, mobile ears. Celine kept subconsciously sometimes running a quick hand over Rumi's ears when her patterns grew, or when a new demon trait appeared, or when they talked about the Honmoon early on, or when Rumi got hurt or at risk of getting hurt (because having those traits would hurt her, and Celine does not want her kit to get hurt). Demon Rumi's ears are pointy and twitchy and may even have a bit of fur on them.
Kids are noisy, but whenever tiny Rumi yipped or yelled in a very specific way Celine would twitch, in the same way she'd later twitch when looking at Rumi's patterns. So Rumi, again, subconsciously learned to associate those noises with being demonic, so as an adult when she lets the demon out she also instinctively makes those noises again. Those are fox noises. (And ooooh, painful thought, what if the reason Celine cannot look at the patterns is the same reason she cannot look at her wrists directly. They are a threat. A sign of Hurt. A sign that the Hunters need to lock up the demon child to fix her. Celine cannot face that about herself with her wrists. She CANNOT face that about her baby with her patterns. Her baby must hide, so she'll be safe from the hunters. This is a contradiction, as Celine, the only hunter left, is the one protecting Rumi from The Hunters. This is not a contradiction Celine will truly be able to realize and face until her kits find a hidden door in the basement)
Celine looks especially hard (kinda contradicts the last one but bear with me here) at one particular pattern in one particular shape on Rumi's arm as a kid. She has traced it a time or two, when Rumi was still young enough for Celine to helo her bathe, or when patching up teen Rumi's wounds. But she always traces farther than the pattern actually goes. That pattern grows to match the red/orange ones on the arm of Celine's mother, aunt and grandmother, not that they'll figure it out maybe ever.
Just. The generational trauma shaping Rumi's demon form, and making her be a little closer to Celine's side of the family.
Tagging @fakelawyerbug @zzzzz-cff @secondtolastrow @frozenwolftemplar