It should be fairly obvious that everything in this post is a spoiler.
I found that every single "single-person" Knight identity runs into logistical problems and/or contradictions. As such, I propose the following:
TL; DR: The Knight is an amalgam, similar to Undertale's amalgamated monsters. Created in a similarly weird underground lab, shrouded in mystery and connected to dr. Gaster.
- Horns
Have a look at their horns. We know that the Knight is a Lightner, and we've never seen Lightners' actual body parts undergo much change in a dark world. Hell, adults seem to even keep their clothing intact. These horns cannot belong to a Holiday as all of them have simple short horns with a small cross. Much unlike the Holidays, the Knight's horns have a very distinct curve and are fairly long. For comparison:
Now, Dreemurr horns, on the other hand:
The overall shape looks like something one would expect from a Dreemurr, but with the weird cross element of Holiday horns. Claiming it's Asriel, Carol or Dess would completely disregard a valid similarity with the other. Claiming it's anyone else is equivalent to just saying the horns are a red herring, which I find a bit disingenuous. The horns are there for a reason, surely.
- Torso.
This image and many others suggest the Knight is anorexic. Well, if they are a regular monster, that is. The part underneath their chest is barely thin enough to fit a spine, and while monsters don't really have skeletons (except dedicated skeleton monsters), their shapes are more or less consistent and they wouldn't be able to fit in such a narrow part of the armoured suit.
Papyrus, however, resembles the lower body of the Knight pretty closely, and, indeed, only has a spine in that area of his body. Yet, as mentioned above, him being beneath the mask would completely disregard the horns that are there for a good reason.
- Hands.
The Knight's hands contain holes, much like Mysteryman's, establishing a connection to dr. Gaster. Clearly, though, the infamous doctor isn't the Knight as our timeline-traveling mystery wouldn't run around Hometown kidnapping people in broad moonlight.
- Timing.
Asriel and Dess have been long gone by the time the Knight actually makes an appearance, but Papyrus disappears the exact same day we come face to face with the Knight. However, if they are actually made on Saturday, that would pose the question of who exactly was the Knight for the Queen and the King?
From Chapter 3 we know that Kris is fully capable of being a "Roaring Knight" insofar as to seal deals with the main antagonist of a chapter (Tenna asking Kris about "their deal"), open a fountain and manipulate the events of the dark world. Chapter 1 and 2 dark worlds are also awfully similar to what Kris does when opening 100% confirmed Kris-brand fountains
- Shapeshifting.
The Knight freely becomes whatever they want, going between a bird, a ball and the actual knight shape. No other Lightner even came close to shapeshifting, unless you count the bossfight at the end of Chapter 2. The most damning piece of evidence, though, is the roaring sprite.
This disregards known laws of matter pretty hard. Clearly, the Knight is a master of shapeshifting. This is a trait shared with amalgam monsters, as all of them seem to treat shapes as a vague suggestion.
- Combat.
The Knight's attacks generally resemble Asriel's (swords and stars), and so does their trail effect. The sound effects of the more magical (that is, non-sword) attacks resembles ice magic from Chapter 2, while flamboyant sword attack poses indicate youthfulness and folly. The Knight is doing over the top gestures and clearly enjoying the battle.
To sum it up, the Knight is a shapeshifter capable of regeneration and possessing traits of multiple missing monsters. They clearly came from the bunker, or have a heavy connection to it, as well as a connection to Kris. They cannot be traced back to ONE person as doing so would disregard all the evidence towards other potential knights.
Everyone in Hometown knows that something really bad happened in the bunker, and nobody wants to talk about it. Some mysterious and influential caller, likely Carol (or someone with an authority even above hers), is even working to keep the whole thing under wraps.
We have seen this before. Monsters disappearing in an event that everyone would collectively agree to forget about. Messed up labs with painful secrets of the past. And the amalgams themselves, simultaneously resembling all the missing monsters. After all, the amalgams's creation was an important event to Undertale, so one would expect something similar to have happened in this alternate world as well...
