Sans's lament.

Sans watches Frisk (aged up of course, to 18) pass through the underground, again and again, making different choices each time. One timeline, they're the happiest couple alive, another, simply best friends. Of course, she's also sometimes a relentless killer. Sans watches in pain, seeing everything he once had be ripped away from him. Sans assumes Flowey is the one to blame.
OK, of course this is Sans X Frisk because I'm a soppy piece of Undertale trash. No NSFW.