IAE says "Hello! So this story is based off of a mod idea that I had for Undertale. All characters belong to their respective owners."
You finally kill Sans pushing you level 20. Where you should be at this point in the game. Maxed out in all stats there is nothing can stand in your way. As you walk towards the throne room you wonder why you let Frisk SPARE their lives in the first place.
Papyrus is loud and obnoxious like a three year old.
Toriel is to caring for her own good and overprotective.
Asgore while in a better mental shape than before. He is still broken inside.
Flowey/Asriel came to the surface after a while but he had no soul. So he was still murderous as ever.
Mettaton was so focused on fame and fortune that you never saw him or his relatives. Though you did hear rumors that her and Napstablook where dating.
Alphys and Undyne are anime obsessed and completely engrossed to their jobs as a scientist and a physical trainer so you only see them on holidays.
Sans was as lazy as ever on the surface though he somehow became Frisks boyfriend.
And I AM A ERROR? He is the weirdest human you have ever met. He is extremely lazy though he can't compete with Sans for the title of most lazy, he is extremely powerful, and yet he seems to be extremely kind to others. Shown by him being the first one to give the monsters homes. But. Nobody around knows anything about him except that he is really good in science which is a little odd.
But you the demon inside every human got sick of all of this kindness. You would have prefered taking I AM A ERROR over because then you could destroy everything with ease. But every time you try to you get repeled by something.
So you took over Frisk and did a TRUE RESET. After you did it Frisk tried to take back control so you killed them and used their DETERMINATION to revive your old body and left hers for dead.