Flame is the final boss of the Genocide Route.
Flame is the only pony monster in the game, maybe not even being a monster.
Flame had followed Chisp, and killed everything they missed in a Genocide Route.
She then admits what she done in her battle, and then says she is missing someone.
Once you kill her, She melts away, then screams that she doesn't want to die. Then, Flowey shows up, saying that he's happy for what you've done, but then you can either kill him or spare him.
If you sold your soul to Omega Flowey, Flame will not fight you, but instead, Flowey will.