so uh
At the end of a generic fairytale, a hero defeats a dragon and saves a princess. Now that might seem random, but when you think about it it makes sense. The hero in the story who is often a knight, could be the Roaring Knight. The dragon in the story is obviously Susie, as she is literally called as such by Gerson. The princess could be "the girl" from the prophecy, who is likely to be Noelle.
I think the final encounter with the Knight will be a one-on-one battle between Susie and the Knight, where Susie tries to defend Noelle from the Knight. This also fits if Carol or December are the Knight, they could be them trying to "save" her from Susie. Considering a battle in a story like this often happens in the dragon's lair, it could also mean a dark world in Susie's home.
This could also be reflected in the weird route in a different way. Kris is also a knight, and called one in their stats. The final encounter could change from the Knight versus Susie trying to defend Noelle, where the Knight tries to "save" Noelle by perhaps indoctrinating her and convincing her to join them, into Kris battling Susie who is trying to protect Noelle after learning what we have done, while we try to "save" Noelle by controlling her to make her into a mindless weapon for us to use.