Geno x Reaper? Like, from that one famous MC roleplay series years back? Honestly, you'll find the bulk of the dedicated stuff scattered across the internet. Tumblr's always been the weird heart of that particular ship for me. You get these amazing mood boards and headcanon threads that build out the world more than some fics do.
AO3 is the obvious archive, but tag filtering is crucial. The ship tag is a bit of a mess mixing different fandoms—some for 'Overwatch' and others for the RP characters. The best stories often get buried under art and shorter pieces. I'd start by filtering for complete works over 10k words; the longer ones tend to be the serious explorations of their messed-up dynamic.
Wattpad has a younger crowd's take on it, which can be hit or miss, but sometimes you find a surprisingly angsty gem that leans hard into the supernatural aspects. I wouldn't rely on it as a primary source though. Honestly, the most interesting meta and prompts still circulate on niche Discord servers you have to dig for.
Man, I've spent way too many nights scrolling for this exact thing. The best ones for that emotional gut-punch usually aren't the explicit romance fics, in my opinion. They're the ones that treat it like a psychological thriller, exploring the absolute horror and tragedy of Geno's loop from Sans's perspective after he's become the Reaper. There's this one on AO3 called 'dust to dust' that just wrecked me—it's all from Reaper!Sans's POV, and the emotional depth comes from his fractured memories of Geno bleeding through, his grief over what he's become, and this horrifying hope that maybe this next reset will be different. It's less about sweet moments and more about the profound sadness of an eternity where your only constant is the person you're doomed to kill.
Honestly, a lot of fics play it for angst-fluff, which can be great, but for real depth, I keep going back to stories that lean into the inherent cosmic horror. The good ones make you feel the weight of the timelines.
That's actually a pairing I haven't seen explored deeply all that often, which is kind of a shame. The dynamic is inherently about a profound betrayal of trust—Reaper literally killed Geno in the game 'Undertale'. So any story that gets them in a room together has to navigate that foundational violence. The interesting fics I've stumbled upon don't just handwave it; they make Geno's justified paranoia the central conflict. It's less about romance blooming and more about the exhausting, minute-by-minute calculation of risk. Does he look away for a second? Does he accept a cup of tea? The trust isn't rebuilt with grand gestures, but with painfully slow, silent proofs, like Reaper consistently choosing not to use his powers even when provoked. It makes for a tense, almost psychological horror kind of read, which fits the source material's vibe perfectly.
Some authors flip it, making Geno the unstable one who has to learn to trust himself not to fall back into a murderous reset loop, with Reaper as an unnervingly patient anchor. That angle feels fresher to me than the more typical 'redeemed villain' trope.