What Emotional Conflicts Define Sans Undertale AU Fanfiction Plots?

The classic one that's always intrigued me is the push-and-pull between vengeance and forgiveness. Frisk, by their very nature as the Pacifist route player character, embodies mercy. Sans, especially after losing his brother, carries this profound, weary cynicism. He's seen timelines reset, watched his friends die over and over. So you get this fantastic tension where Sans is constantly waiting for the other shoe to drop, for Frisk to finally show their 'true' violent nature. It's not just about trust, it's about fundamental worldview. Can Sans allow himself to believe in a genuinely good outcome? Frisk's persistent kindness becomes a quiet, almost painful challenge to his entire philosophy. I've read a few where Frisk isn't just passively good, but actively tries to break through his defenses with stubborn optimism, and Sans responds with increasing exasperation that slowly cracks into reluctant affection. That emotional whiplash, where one moment they're sharing a bad joke and the next Sans is giving them that hollow, empty-socket stare, questioning their motives... it's potent. It allows for a really slow, fragile healing process, if the author goes that route, or it can spiral into a beautifully tragic mutual destruction if they lean into the angst.

That's a pairing I keep circling back to, maybe because the core dynamic is so simple but the variations are endless. The emotional engine is basically a clash between two deeply broken systems trying to understand each other, but neither has the right software. Classic Sans is numb, detached, has seen it all and decided none of it matters. Underfell Sans is all jagged edges and performative aggression, using rage to cover a well of insecurity. They're both protectors, but their methods are philosophical opposites. One sees violence as pointless, the other as a language. The real meat for me isn't in epic fights, but in the quiet moments where those philosophies fail. Like, Underfell Sans trying to provoke a reaction and getting genuine, tired pity instead. That fury meeting a void is just... potent. Or the flip side, where Classic's apathy cracks because someone who looks exactly like him is so openly, desperately hurting, and he can't logic his way out of that mirror. It forces both to confront their own coping mechanisms. Is nihilism any healthier than rage? Is pacifism just cowardice? The fanfics that dig into that, where they become this messed-up mutual therapy session, are the ones I save. The ship works because it's less about romance and more about two halves of the same miserable coin trying to become a whole person.

The sheer volume of alternate universes for Sans is honestly overwhelming at times. I've been browsing through Archive of Our Own for years, and the way writers use the AU framework to isolate and amplify specific emotional states in him is kind of fascinating. It's less about changing his core sarcasm and more about changing the cause of it. Take the classic 'Underfell' setup. There, the surface-level aggression is turned way up, but the best stories I've read dig underneath that. They portray his sharpness not as casual laziness but as a constant, frayed-edge vigilance. The humor becomes a weapon, not a shield, and you get this exhausted, bitter anger that's so different from the original's melancholic detachment. It explores the emotion of sustained resentment in a high-stress environment. Then flip it to something like 'Handplates' or any of the sci-fi 'Gaster's Lab' AUs. Those often zero in on a deep, clinical dread or a fractured sense of identity. When Sans is written as an experiment or a created being, his emotions spiral around themes of belonging and existential fear. The laziness can morph into a depressive shutdown, a literal inability to engage with a world he feels wasn't made for him. It's a more inward, numb kind of pain compared to Underfell's outward rage. And you can't talk about emotions without the 'Horrortale' variants. They're masterclasses in grafting pure despair and trauma onto the character. The emotions explored there are visceral—a survivalist's panic, the guilt of failure, the erosion of hope to a bare, sharp sliver. His jokes become hysterical, a last-ditch effort to keep from screaming. It's brutal, but it shows how flexible the AU model is for stretching a character's emotional range to its absolute limits, far beyond what the base game could ever show.

Man, reading Sans and Frisk stuff always hits different. The emotional core usually isn't about romance for me—it’s about this weird, cosmic guilt. Sans remembers every reset, carries that weight, and Frisk is the one causing them, intentionally or not. Good fics dig into that dynamic: Frisk trying to befriend this guy who knows they could wipe his memory with a button press, and Sans trying to trust someone he’s seen kill everyone he loves in another timeline. It’s less fluffy and more about the quiet horror of pre-knowledge. I lean towards fics where Frisk is a bit older, not a naive kid, and they have to verbally work through that moral minefield. The moments that stick are when Sans breaks his ‘cool dude’ act, not with anger, but with exhaustion. It’s a specific brand of angst. Some writers go the fluff route, which is fine, but the tension evaporates for me if the conflict is just ‘Sans is tsundere’. The best ones I’ve read make the game mechanics part of the emotional language—LOADing and SAVing as metaphors for regret and moving on.

Undertale fanfiction focusing on Sans AUs has pretty clear lanes. The ‘fell’ variants dominate—anything where Sans is a more hardened, often cruel version of himself, like in ‘Underfell’ or ‘Dusttale’. Readers love the moral conflict of seeing a character usually defined by lazy humor and secret compassion turn into someone ruthless. It’s a power fantasy with emotional stakes. On the flip side, the ‘swap’ and ‘caretaker’ themes are huge. Think ‘Swap’ AU where Sans takes Papyrus’s more naive and heroic role, or stories where he’s forced into a protective, almost parental role for Frisk or a human. This lets writers explore his softer side under pressure, which hits different when you know his canon cynicism. Crossover fusion fics also pop up a lot. Sans gets dropped into other universes—‘Harry Potter’, ‘My Hero Academia’, even ‘Marvel’—because his sarcasm and OP abilities (Gaster Blasters, teleportation) create instant friction and humor. They’re less about deep lore and more about the fun of watching him outsmart a completely new setting. A smaller but passionate niche revolves around ‘Gaster-centric’ or ‘backstory’ AUs. These dig into his possible creation by W.D. Gaster, experiments, or a past as the Royal Scientist. They’re often slower, angstier, and attract readers who want mystery and tragedy over action. The sheer volume means you can find almost anything, but the core appeal is always twisting Sans’s two defining traits: his laid-back front and his hidden, weary depth. Whether you amp up the edge or strip it away for vulnerability, that’s where the stories cluster.

Undertale AUs already bend the rules of the original game, but some Sans-centric fics take that meta-awareness and crank it to eleven. I remember one where Sans wasn't just aware of resets—he was actively, maliciously editing the save file itself, locking other characters into loops for his own amusement. It flipped the whole 'depressed, tired skeleton' trope on its head into something genuinely sinister. The twist came when he realized he wasn't the only one with admin privileges; a human player from our world, reading the fanfiction, started using console commands to fight back. It got weirdly fourth-wall-breaking in a way that felt fresh for the fandom. Another common but effective twist is making Sans the 'anomaly' instead of Frisk or Chara. In one popular AU, every reset erases everyone's memory except his, but in one loop, he starts remembering things that didn't happen in any previous timeline—echoes from other, similar AUs. He has to figure out if he's glitching, if there's a higher power messing with him, or if the multiverse is literally collapsing into his universe. It uses the established AU culture as a plot device, which is pretty clever.

Some stories treat Chara and Frisk as two halves of a single soul wrestling for control, which sets up this constant internal war. You get fics where Frisk is trying to be this merciful pacifist but Chara’s memories of the Underground’s cruelty keep bleeding through, pushing them toward violence. It’s not just about being good or evil; it’s about whether the desire for a happy ending can survive the weight of all that inherited trauma. What hooks me is when authors dig into the aftermath of a True Pacifist run. Like, Frisk saved everyone, but Chara is still there, a silent passenger who remembers every single reset, every time Flowey killed them all. That resentment—the idea that Frisk gets to be the hero while Chara is stuck as a ghost of a dead child—fuels so many angsty, psychological pieces. The conflict isn’t resolved by a fight; it’s resolved by awkward conversations on the roof of New Home at 3 a.m., or by Frisk finally admitting they’re scared of their own reflection. A lot of newer stuff plays with the idea that Chara isn’t inherently demonic, just deeply, understandably messed up. Their emotional logic comes from a place of betrayal and pain, not cartoon villainy. That makes the dynamic way more compelling than a simple possession story.