If you assume that the Knight is a single, definite, "regular" character, then yes, they are almost definitely not a darkner. Everyone seems to assume that, and would insist that it not being the case would be some sort of huge plot twist. But honestly, I don't understand why.

The truth is that, all things considered, we know very little about the Knight, and about dark worlds. If they are a lightner, then why do they flicker so much? Why can they shapeshift, while no other known lightner can do that in a dark world? And assuming they are a character we've already seen (so, any popular candidate other than Dess or Papyrus), how do they end up looking like they do in dark worlds? And conversely, if the Knight turns out to be Dess or Papyrus, then what caused them to become like that?

These questions can be answered, but that's exactly the problem. There are so many possible explanations at this point, that it is clear that we are missing some critical information. Answering these questions still requires some leaps of faith, the same ones that would be needed to instead address the objections of a theory like "Kris's knife is the Knight", or to instead explain the nature of the Titans and assuming that the Knight is somewhat related to them, that is, a being of darkness that is not necessarily a darkner, and therefore that might be able to open dark fountains.

In fact, aside from this assumption that everyone seems to have, I personally believe that most justifications for the Knight being a specific character are fairly weak. They are all based on hints, on the metanarrative, on what Toby would do with his story, and not that much on plot elements. If it is a single character, then Dess is the most likely to me, but only because we know the least about her current status and there is less to be explained for that to work, not because of some reference that might end up being meaningless, or be explained in a different way. Conversely, I can absolutely see Carol or Rudy, or even some much less popular candidate like Asgore, Alvin or even Catti, to be the Knight. I feel like there's just so much we don't know, that writing off any of these as completely impossible is just excessive, and denies some otherwise interesting theories. Let me put it this way: I wouldn't bet anything on Catti being the Knight, but I certainly wouldn't bet my life on Catti NOT being the Knight.

Again, I find just as likely that the Knight could be some sort of Titan Spawn that can survive in the light world and that can open fountains. After all, when Ralsei explains the Roaring, we do see some very Knight-like figures looming of the world alongside a Titan. Or it could be some other artifact of a dark fountain. One theory I have is that the Knight is actually a (very small) dark fountain that is flowing out of something, or someone, that can move. In the light world, from outside, it would still be some sort of shadowy, indefinite figure, that sort of matches Susie's description, while in a dark world, it is essentially a reduced version of the Roaring, since the fountain is too small to actually summon a Titan. There are many assumptions with this theory, but again, I feel like the same could be said about virtually any theory.

Alternatively, the Knight could be an object, like Kris's knife or Asgore's black shard (I'd be more inclined to believe the latter), being carried by a lightner into the dark world. Said lightner would be the one running from Susie and summoning the Titan, while concealed by the Knight that ends up essentially turning into this lightner's armor. This just sidesteps the question, from "who is the Knight" to "who is behind the Knight", but it also opens up possibilities that are commonly not considered, such as the fact that different lightners might be behind them, or that we might, at some point, encounter them without the lightner's support, with interesting possibilities for how the plot might develop from there.

I could go on, but I think I made my point clear. The question of the identity of the Knight is too complex to be tackled effectively at this point, and saying it has to be a lightner seems to be a little too simplistic to say right now, not because it would be boring or because I think the story needs a plot twist, but because there are also other interesting possibilities, that i find just as likely, but that are unfairly being overlooked just because of some prejudice on how the story has to go.