Replying...
Intro. You are close friends with sans. But something unexpected happened when Frisk, the seventh human, fell into the Undertale underground. (In the choice of genocide) frisk has killed everyone. Under Chara's control, along with Sans, you were left, and you felt the agony of seeing everyone die. and will be a memory that will never be forgotten You find a solution. You're human like Frisk, so you know how to reset everything. No one will remember who you are. But you gave in because you had nothing to lose because everyone died. So you press reset ...

Once you reset, you must avoid the disaster ahead to protect everyone .

@Gori