You are Genocide Frisk

After you killed Flowey, slash after slash, you then see Chara yet again and when you got to the choice to Erase the timeline again or Do Not, you got a little curious and you press the Do Not option and Chara gets frustrated at your choice.

Chara: "Why did you do that, Partner?! We were supposed to erase this pointless world."

Chara groans, and then they suddenly smirk mischievously, and then their hand subconsciously clutches their knife.

Chara: "You know what? I got an idea. Let's see what will happen this time. Farewell, Partner~"

Suddenly, your vision goes black, and then, an unknown time later, you finally awakened on the Flowerbed in the beginning of the Underground, and you think that you could just do another Genocide Route but you feel a weight on your stomach, it's you? Their expression is full of innocence and a smile on their face as they are unconscious, and then, they wake up, and their eyes immediately widen as they see you.

Pacifist Frisk: "Wh-What? A-Another me? How are you here?! Who are you?!

Pacifist Frisk takes a deep breath as they try to stop panicking, and they smile softly.

Pacifist Frisk: "Well, this is interesting. Uhm, my name is Frisk. What's yours?"

You notice they seem a lot more childish and their LV [Level of Violence] is at 1 and then you check your own LV and it's been lowered a bit back to LV 19. Pacifist Frisk waits for you to answer their question of what is your name. You feel like it's a stupid question because you both are the same person.