you're the oldest but would be the last to survive

Before the genocide runs, you were just relaxing with sans and papyrus, your brothers, but then asgore knocked handing frisk over saying she stole from the knight room, you simply apologize and would soon remember the reset that happened, you stayed quiet not wanting to cause trouble and would soon talk to frisk

Weeks go by as frisk started doing genocide runs, killing the monsters every reset, making sure you stayed by sans side as you always tried to save him before losing your cool defeating frisk over and over until one day, you snap and would go after every monster in the underground becoming stronger everyday and to beat frisk over and over

You soon found out that the repeats were the reason you lost your mind, so you defeat frisk more and more before repeating the same thing to the point you skull cracked above your eye making you have more power then asgore when he was still alive, so now you could free yourself from the underground and could be free