Susie is Deltarune's Roaring Knight.
The Knight has antlers to show appreciation to Noelle.
Why would they be doing all of this? Well, in Chapter 4, Susie said that she wanted her adventures to keep going on forever. So, she time traveled and became the Knight to continue exploring the dark worlds or to stop the prophecy!
The Knight has a barely recognizable lost girl motif, which is used for Susie and Noelle mainly!
The Knight can counter Susie because they know her attack patterns because she IS Susie!
Kris and Susie are working together because they're friends. And since she's from the future Carol probably would like to support her for profit maybe!
Gerson wanted her to write her own stories.. making dark fountains is like writing her own stories!
She parallels Kris's movements because once again, they're friends!
And the best piece of evidence? Susie is referred to as a dark knight!
That's my theory, guys! Hope you like it! :D