Okay I've seen tons of people on the Dess Knight theory, but I've had some bonkers ideas of my own!
Idea 1: The Knight is Kris' knife.
Okay hear me out- The Knight appears in multiple dark worlds without ever turning into a statue, which implies they're a lightner, but what if they're actually an object that Kris carries around?
We know that Kris doesn't use their knife as their sword, instead using a pencil. This leaves the knife technically unaccounted for.
TRK and Kris have similar mannerisms, often striking the same poses. This would make sense if TRK was an object the belonged to Kris. It could also account for their high damage and jagged appearance.
Also, just to really rub it in, the theme that plays when fighting TRK is "Black Knife". Most of their attacks involve blades, cutting, and slicing.
While I won't get into this too much here, Kris does have a theme of self harm, so for their greatest foe to just be them hurting themselves would unfortunately be in character. This would also tackle any implications of Kris working with TRK.
Before the Titan is summoned, Susie says this
Which TRK laughs at her for. If TRK were the knife and connected with the themes of SH, this would be TRK laughing at the irony.
Idea 2: The Knight is Kris' sweater
Again, this would make TRK a darkner inhabiting an object Kris carries with them. No other lightners seem to have the clothes they wear turn into darkners, so why would Kris' sweater be different? Because it isn't their sweater.
We, The Player, know this is Asriel's sweater. This implies Kris wears hand-me-down clothes and not clothes that are actually theirs (This is also supported by the many different fits the wear in the dark world and them having a different color scheme in the dark world, unlike other lightners. (Not counting Noelle, but she's themed after her home)
TRK being the sweater would make them a manifestation of Kris' feelings about their older brother. TRK's appearence could also allude to a connection with Asriel, the horns, tall build, broad shoulders, etc, while also being twisted and sharpened by Kris' negative feelings.
This would once again tie into the themes surrounding Kris, this time being them always living in their brother's shadow, never quite good enough to to be the main focus of their own story, villains do get all the attention, after all.
For TRK to be defeated, Kris would have to stop living in their brother's shadow and be able to make a name for themselves. Unfortunately, with us around, that's going to be quite difficult.
Idea 3: The Knight is yo mama
Sorry I had to. But anyways, I do have a few things that require explaining if I'm saying TRK is a darkner.
How can TRK take Undyne through the light world if they're a darkner? Carol.
If TRK is an object that's always with Kris, why do they not appear in chapters 1 or 2? As the chapters go on, Kris is seemingly becoming more and more upset with everything going on. If TRK is a manifestation of Kris' negative feelings (channeled my an object), then they would appear more and more as their mental state worsens.
Why are they The ROARING Knight then? Idk I got nothin
Do I sincerely think either of these are true? Nah not really. I'm a Papyrus Knight truther.
Actually though, I think Dess is the most likely theory. For as much as I'd love for any of my theories to turn out true, there's just a lot more evidence for TRK being Dess than any of my wacky theories. That won't stop my fun though!