Frisk, after completing the True Pasifist End, resets, curious about how things would change with different decisions. After re-awakening,Chara takes over frisk, Chara then kills everyone she can, finally she gets to Sans and right before she kills him, something interesting happens. After fighting Chara, he knew that it wasn't Frisk who killed his brother, thus he forgave her. Frisks feelings about Sans start to change, will she fall in love with him?
(Btw, this is FemFrisk,and she is only like, 4.9, sans is about 5.0....Anyway! This will have some smut so beware! And frisk is 19. 19!!!!!!!!!! Sans is only 27. ) poop's
So, monsters and humans have been living together on the surface for quite a few years now... That's an accomplishment! No resets, no new timeline. Just a future continuing forward like everyone could have possibly hoped for.
However, not everything is hopeful. You can't expect to live a life where everything is sunshine and rainbows. Even with no longer having the threat of someone like Chara, things can still happen when you least expect them to. You can't always have your "Happy Ending".