Monster Tinder

[Amaterasu]

Sans

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Kobold!

Well, let's be real - Sans x Toriel will always be a thing. But, I think that we can offer our skelebro an alternative as well. And that alternative is Kobold.
Kobolds are a sort of good-luck monster who typically takes the shape of a bipedal canine, though Kobolds in other forms exist too. They all have in common that they are physically active, helpful, friendly and social. All of these up to eleven. Most Kobolds are sports fanatics and constantly training. Their workaholic attitude also tends to make them a lot of money.
This is quite opposed to the lazy Sans. Sure he's friendly, social... well, somewhat, and helpful to some degree, but he's more of a thinker than a doer. He has a whole load of issues beneath his nonexistant skin, from being resigned to his fate to suffering from depression. How on earth would he fit well with someone as hyperactive as Kobold? The answer to that is Papyrus, Sans's brother. Or rather, what Papyrus stands for. He's the active, strong, open and naive of the two, much like a kobold, and he's the one keeping their little household afloat. Sans would probably not last very long without Papyrus's driving force. Kobold is very similar to Papyrus in that regard - many of the things that he stands for, she does too.
Of course, Sans might not bother listening to Kobold since she's not Papyrus. Or maybe not initially. That's fine! Kobold isn't dependant on his participation. Her cheerful, open attitude will eventually take roots in his chest, and he will grow to appreciate her. He probably won't participate in her daily 42 kilometer walks and might not listen to her healthy-diet speeches, but there is no risk that his laziness will pass on to her. On the other hand, his intelligence and knowledge will compensate for Kobold's naivety and help her in furthering whatever crazy projects she may pursue. And she's going to appreciate his jokes and puns, too! Well, probably.
And if all else fails and he really just needs his girlfriend to calm down for five minutes, he can just pet behind her ears and she'll melt against him.