Every non-DessKnight theory has 3 insurmountable problems with them
In order of "This doesn't make sense for anyone else except Dess":
Part or All of any potential candidate's actual personality has to be a complete fabrication, or they're revealed to be WAY more devious than they've been shown, to the point where it basically deletes their character. Think Mikan of Danganronpa 2. (Applies to basically everyone except Carol, for whom it does seem at least consistent with what little we know. But Father Alvin? Asgore? RUDY? Rudy Knight is especially hit bad by this, it destroys everything established with his character up to now)
Sheer problem of Who Has the Time To Sit In Closets to make Dark Fountains (so, Not Carol. Arguably, no one with a significant position in Hometown's community, so not Alvin. Also not Rudy either, he is literally bedridden and surrounded by doctors, including a receptionist who is very chatty about confidential patient details. He gets winded after leaving and standing around for a few hours for church service. They'd Notice if he kept going missing for hours on end)
Even if you account for those two, why is the Knight a Fucked Up Dead Deer-Ghost? Absolutely no reason for Carol, and Horrible Implications on the writers' part for Rudy; either his illness is caused by his transformation, is how the Dark World changes his illness into Super Death Powers, or he's just lying. None of these are good and make me want to strangle someone on behalf of my cancer survivor friends
Meanwhile, DessKnight fits every one of these perfectly. We don't know anything about Dess's personality aside from heavy hints that she's set up to be Susie's foil, which sounds perfect for a big antagonist who can have a dramatic moment between it and Susie, while also having close ties with Kris
Plus, the motivation of every non-DessKnight is so shallow: they're looking for Dess, and might be willing to end the world to get her back (or: Just kidding! They're actually playing 5D chess with Fate Itself and the Roaring is just a risk they're willing to take, teehee!) Okay. Sure. But why are they then also slavishly carrying out the prophecy? The only explanation is that they're completely faking the Harbinger of the Apocalypse role as part of some scheme, but how does that make sense next to the Knight's actions in chapter 4? They literally get ragebaited into summoning a Titan. Ralsei is surprised by this, meaning it's not even something that's supposed to happen, or at least not something the Prophecy says will happen (hence he has no idea how to deal with it, except a guess that it could be sealed like a dark fountain). It's what he feared was going to happen in chapter 2 that made him stop the others by telling them about the Roaring
The Knight Does Not Care, and it shows no signs of caring, about anyone except Kris and Noelle, but she's not been involved in a Dark World where the Knight was still present, yet. Clearly it did make Cyber World FOR Noelle, tho. Which, again, fits better for Dess than anyone else
Dess is literally Lost and probably can't really exist properly in the Light World. It makes sense they'd be hiding in dark places like closets, only really moving around at night, taking shelter in a Shelter with a dark world in it
As for why it's Like That, the Black Shard being unchanged in the Light World also explains it, as well as why it's able to overpower Undyne even in the Light World; the Knight's nightmare shadow form is just what they look like now, in or out of the Dark World, and with a power scale to match. And if any Knight Candidate fits to be a Dead Deer-Ghost Monstrosity, it's the Lost, Lost, Lost Girl who has literally been haunting the narrative up to this point, who features the most prominently in the very same chapter where the Knight appears for the first time
There is, as far as I can tell, just one single downside to DessKnight, and that's:
Why has the Roaring not started yet if Dess just wants to end it all (which I believe is its ultimate motivation)
And THAT, I'm willing to Wait and See, cause the answer doesn't need to be Obvious right now, like it is with every other Knight Candidate. We've so far had two characters tell us that the Knight's motivations are more complicated than causing random destruction:
And Old Man Gerson is the one that most strongly sets up Susie and Dess as foils:
Like, it's obvious he's talking about Dess here, and not JUST a fictional character from his story, right? Look at how pensive he looks, as if remembering someone with a sad fondness. It's highly likely that, given Gerson is said to have passed away "a few years" ago by his son, that he would have been around when Dess disappeared, for which it has been ALMOST a decade
And, all of this is before the final encounter with the Knight in 3rd sanctuary, where they basically impulsively almost start the Roaring, even if there wasn't any obvious way to stop the single Titan. Sure, maybe they knew the Old Man would bail the heroes out, but it was Susie's Idea that saved the day, not just the Old Man showing up. And sure, maybe the Knight could defeat the Titan on its own, but again, if that was a possibility, it would have been brought up., especially by someone like Ralsei
Between the music, the battle flavor text, and again, the fact that only Susie's Idea is what saves the day, it all indicates that even Kris, the only one who actually knew anything about the Knight's actions, thought they were all boned. There's no way this was part of any sort of Plan. So...
Yes, they don't seem to ONLY have a goal that is "Start the Roaring asap" or "Spread mayhem and violence indiscriminately", but they ARE unstable enough to almost accidentally start it anyway, all because the girl who stood up to it more or less dared them to. And yet:
... it still did hesitate when Susie begged them not to take Toriel. This doesn't make sense if this whole attempt to steal Toriel was never part of the plan. Because why have this moment of hesitation? Keep in mind, this scene happens whether the Knight floors the party, or Susie manages to chip its sword. This is the first clue that the Knight will Remember Susie. That Susie is capable of reaching them, where Ralsei's begging them to stop falls on deaf ears
Furthermore, Alphys confirms that Undyne shows up so late because she spent time with her at the church's night service
So if Kris arranged for Undyne to be called to their house as an alternative target, then the whole fight with the Knight really was a stalling attempt because their backup plan of calling a better target had almost failed, and it's only because of Susie making it hesitate, that Undyne was able to arrive in time to grab the Knight's attention instead
This tells me that, while the Knight may not be a total maniac, and does at least pull their punches with the fun gang, and especially Kris, they still were perfectly fine with kidnapping Toriel, and only changed target because Undyne was even more tempting. And she only became an option because Susie's stubbornness was the only thing to make it hesitate.
This is why I think Dess is still fully intending to start the Roaring at some point. But why hasn't it, either out in the open or even far away from Hometown?
Well, Hometown has something that isn't anywhere else; the shelter. Which is the Knight's point of operation, from chapter 3 onwards if not the whole time
Not only is it currently the best candidate for where Dess originally disappeared (which puts a hole in every other Knight Theory, as wouldn't this location be the best place to look for her, and not in closets or abandoned classrooms?) but there is clear significance as to why it's a nuclear fallout shelter, specifically
Again, mandatory dialogue to see, on both routes too
So, if the Knight is operating out of a literal place designed to survive the apocalypse, and is in every way capable of starting it, then it stands to reason that what they're waiting for is the Festival, and specifically the return of Asriel
Because that would mean everyone Dess knew and probably cared about in life would be back in Hometown. Back in the only place where it might conceivably shelter them, even as the rest of the world (and probably the Knight itself) ends
While Susie is named by Fate Itself as the bearer of Hope, DessKnight would be utterly devoid of it. They would embody the thesis of the game, that your choices don't matter, and no one can change their fate. So they might as well try to shelter what few people it cares about away just before wiping it all out
But with her white pen (or white paintbrush, if her weapons having been a number of different brushes is any hint) Susie may be able to compose something new...
... and write a new future for everyone; including a lost, and lost, and lost girl.