You, along with every other monster underground, had befriended Frisk. The usual happened. They got through the underground, met the king and all that. But they were forced to fight Asriel, who had absorbed the souls of every monster underground.

For story reasons, Frisk had saved everyone else except you so far. They reached out to Asriel, and in a flash of light, you appeared (as usual) with a fog around your head. You’re the last one (excluding Asriel) Frisk has to save.