How Do Writers Develop Underfell Sans X Underfell Frisk Character Dynamics?

Interesting question. I think a lot of writers forget that 'Underfell' means 'under' and 'fell'—it's not just edgy aesthetics. The dynamics hinge on survival in a hyper-aggressive AU. Sans isn't gonna be charmed by persistence; he's more likely to be intrigued by Frisk's capacity for violence or their stubborn refusal to die. Their interactions are a power struggle, a constant testing of boundaries. Does Frisk fight back with equal ferocity, or do they use a cold, calculating patience that unnerves even him? That push-and-pull, the ambiguous motives, that's where the character work happens. It's gritty and uncomfortable and that's the whole appeal.

Man, Underfell AUs are all about the friction, aren't they? The whole point of the setting is flipping morality, so Sans and Frisk are both walking disasters in their own ways. Sans isn't just grumpy; he's actively malicious, a bitter sentry who probably enjoys the idea of a human falling into his territory just so he has something to torment. Frisk, meanwhile, isn't the pure-hearted pacifist. They're surviving in a world that wants them dead, so they're hardened, maybe even cruel if pushed. The dynamic isn't about redemption through kindness; it's two sharp-edged personalities forced into proximity, either through circumstance or a twisted mutual interest.

Some writers go for outright enemies-to-allies, where they only stop trying to kill each other because a bigger threat shows up. Others lean into a more manipulative, transactional bond—Sans might see value in keeping Frisk around as a tool or a source of entertainment, while Frisk uses Sans's knowledge of the Underworld to navigate its dangers. The 'ship' aspects often come from exploring the vulnerability beneath all that performative aggression. Maybe Sans catches Frisk in a rare moment of exhaustion, or Frisk sees past his constant smirk to the genuine exhaustion and despair that fuels it. The appeal is the slow, grudging erosion of those defensive walls, built from shared sarcasm and a mutual understanding of how rotten their world is.

I've seen a few fics that completely lose the plot by making one of them secretly soft from the start, which misses the point. The tension is the core. Are they capable of trust, or is this just another game? It's less about fluffy hand-holding and more about two damaged people recognizing a similar damage in each other, even if they'd never admit it aloud. The best ones make you question whether their connection is healthy or just another form of mutually assured destruction, which is way more interesting than a straightforward romance.