Okay, so I was just rereading some older 'Flowey is Not a Good Life Coach' fics the other day and it struck me how many of them hinge on that Frisk/Chara push-pull. The dynamic basically offers a built-in excuse for every romance trope in the book, doesn't it? You've got the whole 'sharing a body' thing—that's instant forced proximity, which writers love. But the more interesting bit is the moral ambiguity. Is Chara a ghost, a demon, a traumatized kid, or the narrator? Fics pick an interpretation and run with it, and the romance plot bends to fit. If Chara's vengeful, you get enemies-to-lovers where Frisk is trying to redeem them. If Chara's the sad ghost who needs help, it's hurt/comfort with a supernatural twist. I've even seen a few where Frisk is the unstable one and Chara's the voice of reason, which flips the whole thing on its head.
What really makes it work for fanfiction, I think, is the massive gap in canon. We know so little about either of their true personalities, especially post-pacifist run. That blank slate means writers can project whatever dynamic they want onto them—childhood friends reconnecting, bitter rivals finding common ground, two souls melding into one entity—without worrying too much about breaking character. The most common thread I notice is the power imbalance, though. One soul holds the other's fate, literally. That can go creepy real fast, but when handled with care, it creates this intense, codependent intimacy that's perfect for slow-burn angst. Sometimes it feels less like a romance and more like a study in shared trauma, which honestly might be more fitting for the source material anyway.