Susie is escapism through material items/Susie is a hoarder (CHAPTER 5 SPOILERS)
Since Susie has basically nothing in the apartment she lives in, she gained attachment to the Darkners, which in the light world are just regular items.
When Susie asks Gerson what character from his story/DragonBlazers she'd be, he calls her a Dragon. And what are Dragons know for being? HOARDERS!
With that being said: The true final boss to Deltarune is Susie herself.
Think about it for a sec. It'd be way to obvious for the final boss to be The Knight or even the Titans from The Roaring. Sure we will eventually have to fight them, but if the the last glass panel from the prophecy we're to involve closing castle town forever, wouldn't it make sense that the person that would fight the hardest for it (besides Ralsei) would be Susie?
In Chapter 5, we see that she decided to live in castle town, most likely forever. She'd most likely give up her own life if it meant that the castle town fountain stayed open to keep the Darkners from going back to just being objects. After all, a dragon has to protect its hoard.

















