New(?) Knight Theory
I don't know if anybody has thought of this or if anyone's ever mentioned it but I have a possibly new theory for the who the Roaring Knight is.
Or in this case, who the Roaring Knights are.
Yes ladies and gentlemen and otherwise, my theory is there are TWO (2) Roaring Knights.
This might seem odd and I know it sounds like im just trying for shock value but I think there is genuine evidence for this.
For one, a common discrepancy among DT players regarding the Knight's identity is their differences in personality between Chapters 3 and 4. Many theorists have noticed that the Knight acts differently between both. 3's Knight is listless and has very slow, serious behaviors. They went into the dark world to grab Toriel and get out and they didnt show the Fun Gang any mercy (besides Kris) whej fighting them.
Whereas in Ch4, the Knight is cocky, confident and even laughs at the Gang. They're constantly aura farming and baiting Susie very intentionally. This is a stark contrast to their behavior in Ch3 where they were very sluggish and intimidating.
My proposal is that these are, indeed, two separate characters wearing the armor of the Knight at separate times in each chapter.
As for who each Knight is: i believe that the Ch3 Knight is Carol (yes, boring, I know, but bear with me here), and that the Ch4 Knight is Rudy.
Now before all the Dess Knight believers jump me, I want you to just hear me out. Dess does have a lot of evidence for being the Knight, but frankly we dont know enough about Dess to really confidently say that shes the Knight.
However I am fairly confident that both Rudy and Carol are each Roaring Knights and there is evidence, both thematically and mechanically within the game.
For starters, in Ch3, the place that we fight The Knight is called the "Cold Place". Now which characrer, specifically is the MOST tied to the theme of ice, snow, and coldness? Carol of course (I mean yeah Noelle too but she's obviously not it). So I think, thematically, its no coincidence that we fight the character most tied to coldness in the Cold Place in ch3.
On top of that, I think that the way that the Knight acts in ch3 also aligns much closer with how we see Carol act, as briefly as that is. Carol as we see her in ch4 is slow, intentional, bitter and gets to the point of what she feels and does without messing around even a bit. A very no-nonsense attitude, naturally. And as I said before, the Knight in ch3 also doesn't mess around at all. They try hard to defeat the Fun Gang, and even if the player is cracked and ends up surviving most of it, Kris naturally eggs her on to just end it since they also know that Carol just wants to get this over with. Hence she puts all of us down and goes back to taking Toriel again. And when Undyne crashes the party, Carol opts to just take her instead. Plus I think its reasonable to assume that Carol would have enough strength to drag Undyne down to the shelter (something that Rudy definitely can't do).
Now as for Rudy in ch4, I would assume that he would take the opportunity to go back to the Church right before it starts raining (while you and Susie are at Noelle's house). After all, the Church and Hospital are less than a block away from each other so he could reasonably just go back and forth when needed (or possibly when told by Carol). Of course, the reason for the Church being a dark world from the beginning is a mystery, but I'll expand on that more in a bit.
Other evidence that could support this is that you cannot visit Rudy or go into hospital at all once it begins raining as the hospital is closed. So we dont technically know whether or not Rudy is there, meaning it could be possible he went back to the Church right when Carol went back home to Kris and the others.
Now another thing that has to be considered when talking about the Roaring Knight is their motivations for wanting to bring about the Roaring. After all, why would a Lightner want to bring about an age of Darkness and nightmares upon the world? Well, I think this ties back to the poor lost girl haunting Deltarune that everybody doesn't know and loves: December Holiday.
Another common theory around Dess' disappearance is that she is quite literally hidden away within the code of Deltarune and is simply wandering around in the darkness trying to find her way back to the light.
So, assuming this theory is also true, how exactly would her parents, who both are also theorized to have distinct knowledge about the dark worlds and the prophecy, try to find her? By going around and opening dark worlds wherever they may find her of course. In her old classroom in Chapter 1, in the computer lab in Chapter 2, and the Church in Chapter 4. All places that are very strongly tied to Dess and even Noelle in some cases.
And if all else fails and they can't find Dess in the darkness within any contained dark worlds, then they'll need to take more drastic measures: The Roaring. If they can coat simply coat the entire world in darkness, then that would surely allow them to find Dess for sure. The rest of the world be damned, they just want their daughter back. That's how they'd see it anyway.
Anyway, that's about the extent of my theory. I'll add more in other edits if I happen to come up with more evidence or note anything else I need to. Feel free to comment what you think and if you think this makes sense or if its a load of crap, I dont care lol. This is my interpretation so far and I think it would be super cool if this was the case. Thanks for reading!