I'm not usually into OC pairings, but something about the Muichiro/YN dynamic keeps me clicking. The conflict feels so embedded in who he is—this prodigy who's mastered a Hashira's duties but is still emotionally a child, haunted by massive gaps in his memory. A YN character, especially one who isn't a demon slayer, creates this immediate friction between his duty-bound, distant self and the simple human need for connection he doesn't even know he's missing. You get scenes where he's utterly baffled by someone caring if he eats or sleeps, and the YN is just trying to bridge this impossible gap. It's less about external drama and more about the quiet ache of someone relearning how to be a person.
I think the best stories avoid making the YN a therapist or a cure. The conflict comes from them being an anchor to a world he's detached from, which sometimes means watching him pull away into his missions or his fog. That push-pull, where he might have a moment of clarity—remembering a fragment, feeling a flicker of something—only for the memory to slip away again, is the real heart of it. It's inherently melancholic, but the small victories, like him starting to recognize their scent or voice before he remembers their name, hit harder than any grand confession.