Doctor Canon's Office

Its design is an exaggerated version of Kris's armor without the scarf. The pauldrons look like angel wings and the helmet looks like antlers. Moreover: the knight is clearly not enjoying itself. It cried before trying to assimilate Toriel in chapter 3.

Kris being able to communicate with it, the knight lowkey knowing the layout of the Holliday house, keeping them from getting the shelter code. Kris is clearly Way In Over Their Head, and trying to help in their own strange way, wanting everything to go back to normal. "Remember Kris, you promised", sounds like something Noelle would say.

When Susie smashed the prophecy in Chapter 4, I figured that it implied that there would always be a Roaring Knight and that it would have to be one of The Party implying that The Tragedy would be a fate worse than death.

Now this theory has plenty of holes and it's really dark for a game like deltarune. Cause that would mean that Kris and Noelle are essentially stuck in a neverending torture loop that Susie is unintentionally perpetuating and Ralsei's known this whole time and just hoping things turn out differently this loop, however many there have been up till now. It still doesn't explain where The Soul came from or what the point of The Vessel is.