It's pretty obvious that the FINAL TRAGEDY is THE GIRL sacrificing herself, and it's pretty obvious that given all Jesuss Christ imagery and parallels, Noelle is meant to be THE GIRL.
Original heroes, were YOU (Vessel), Noelle, and Ralsei (there still a possibility that The Prince, too, meant to be someone else, but we leave it be Ralsei for now), with the original story ending with Noelle dying and the world ending with sans escaping into Undertale.
But then this scenari was hijacked by Knight, Kris, and potentially even Carol.
With Kris stealing your role as THE CAGE.
Main reason he and the Knight did that is to save Noelle from getting sacrificing.
And as a replacement they agreed to use Susie, because before chapter 1 Kris saw her as irredeemable bully, and had no problem with her dying (In ORGINAL sceanrio with Vessel and Noelle, Susie served as chapter 1 main antagonist, with it ending you and Noelle beating Susie or alternatively "slaying the dragon")
But since Noelle or YOU went into the dark world in chapter 1, it didn't play out that way, and now the main conflict is that Kris feels conflicted regarding sacrificing Susie, since he now knows how good of a person she is.