it means sans bones

I don't think Dess is the Knight. She's a character defined by her absence, in a story where toxic nostalgia and refusing to let go of a past that isn't coming back is a main concept, and to be honest I don't think she's coming back at all.

The knight is Rudy, aided by Asgore, looking for missing Dess.

So why Rudy?

Tl;dr - Deltarune (and Undertale) has shit dads as an ongoing theme, so it makes sense that the main villain would be A Dad.

This is a lot of conjecture on my part, but it works for me narratively:

He's the only character we see who is familiar with every version of the prophecy, from Lord of the Hammer (presumably being in the parents' generation he was taught by Gerson), to the church version (we know he used to go regularly, Carol doesn't), to Dragon Blazers which he plays with Noelle. The Knight is familiar with the prophecy to the point of obsession, particularly with "the girl" as we see in ch.4 (with the church reflecting the Knight's interpretation, because they made that dark world), and is certain it will come to pass.

He's the "fun parent", the one who we can assume engaged his daughters in escapism while Carol tries to shield them from the world. If the kids used to go to dark worlds in the pre-game past (which we can surmise from Ramb's dialogue in ch.3), it's not unlikely that he's the one that introduced them to the concept. If Dess is lost in the Dark, he's the one who'd know about it, and the one who'd feel guilty about it. Kris, being the only human in town and necessary to close the fountains even when they were younger, would also know this.

The Knight knows all the dark world lords, knows their baggage and how to tempt them, presumably based on prior play with the kids. This could go either way with Rudy or Dess, until Gerson. He manifests very clearly in the church, which means the Knight would have had to know him pretty well when he was alive, more than someone who'd only a few years older than Kris would have.

The hospital is a perfect alibi, and is conveniently inaccessible to us any time the knight would have been out creating a new dark world. And the Knight's body looks pretty wrecked, doesn't it? (The idea that he's sick and gets worse because of how he uses up his body in the dark worlds is also an interesting one.)

This bit of dialogue difference if Rudy is not introduced to Susie in ch.2 (in this case, his first encounter with her would be when fighting her as the knight, not when learning she's Noelle's crush)

Just narratively, the fact that he's there in every chapter, in plain sight, seemingly frail and harmless, his chill & disarming demeanor, in a series of works all about fathers doing terrible things to put their family back together, even if it's no longer possible... it's the most obvious choice for a tragic villain for me.

Kris and Asgore both have the agenda of bringing the Dreemur family back together, and we know the Knight is working with Kris and that Asgore has a shadow crystal so he must be as well... and we know Asgore visits Rudy daily to bring new flowers.

This is my interpretation:

The Knight is obsessed with "The Girl". We see her image over and over and over and over again in the church.

Why would the Knight, who regardless of who it is would barely know Susie, be so single-mindedly focused on The Girl? If it's a dad looking for his missing daughter, that would make a bit more sense.

In my reading, the "original" trio of heroes (according to the knight's version of the story) was supposed to be Kris as the human, Dess as the girl, and the Noelle as the prince in the dark, whatever that means in that context.

The prophecy image for the Prince and her sprite in the mantle game are pretty similar, and this 3rd hero is the only one we see using magic rather than a weapon.

Also note that in the ch.4 prophecy it says "Alone in Deepest Dark" in the Knight's version rather than "FROM the dark" in Ralsei's version, meaning the Prince isn't necessarily a darkner. I don't even think the Knight takes Ralsei into any consideration. (Thinking about it now, this could also refer to Dess lost in darkness, in which case Noelle would be the girl, but still it's the two of them and Kris)

The Roaring is the catalyst event that will draw out the heroes, and in the Knight's perception the prophecy must come to pass - therefore the roaring will have to draw out Dess back from wherever she is.

To the Knight's plan, Noelle is meant to be part of the story, so he needs her in the dark world. The Knight wants her to grow stronger, and at the end of ch.2 Queen asks her (presumably at the instruction of the Knight) to create a dark fountain inside the dark world that would bring about the roaring.

Susie, 'the dragon', is meant to be killed by Noelle as per the prophecy, and that's so heartbreaking. We might see that path in Weird Route.

The game is all about agency and whether or not you can challenge your place in the world and your fate (ahh, Spamton, you kill me...), and Susie is the biggest driving force of it. We now see the prophecy diverge into two different interpretations, the one where Susie is the hero and one where she is the dragon. She will likely choose a 3rd path that breaks away from the roles assigned by the prophecy.

(Where we also see these interesting silhouettes as the titans in ch.2... Clearly two Knights and in the center, perhaps, a King?)

Asgore's secret ch.4 dialogue implies, to me, that the original intention of ch.3's failed Toriel kidnapping was to take her to the shelter to prove something to her regarding Dess, which she refuses to partake in willingly, so he has to contrive a way with Rudy to do it by force.

Asgore has free access to the shelter as former police chief, and also? Probably helps carry Rudy around town, run to the shelter, etc. In ch.5 we're going to see what he's trying to hard to convince Toriel and everyone else of. This is probably going to take place during the Festival. They'll have no choice but to participate this time.

Also credit to my partner for pointing out, if any of my reading is right, we're gonna see a huge flood of Rudy/Asgore shipping.

Rudy and Carol are on opposing sides of the theme of nostalgia. Carol is on the side of preservation without change, keeping ephemeral things frozen in time (the snow flakes cast in bronze, Dess's room).

Rudy as the Knight is on the side of tearing open the past, diving into the old memories, and shredding them apart in the process of trying to move forward and make the prophecy come true.

The Weird Route, the path of making Noelle stronger, is the extreme of that, all about destroying those connections to the past - by killing darkeners you're literally severing your childhood play imagination from your present self.

If we're lucky, we're gonna get to see Carol fighting with her katana against the Knight! ty @luuton-ao3 for the amazing idea!

And my final and best prediction:

In the final confrontation between the fun gang and the Knight, Rude Buster will turn into Rudy Buster. My partner and I had this realization joking around on Christmas morning. (I'm not from a christian culture and have never celebrated before moving to his country lmao so all the DR christmas references are utterly meaningless to me)