I really don't think the knight is literally just a lightner from hometown

Mainly due to its physical appearance, we see what's under the armor, nothing besides some snake eye thing?

I think it's pretty accepted that the knight may be lighter-adjacent, with something going on. I, however, think its "base" is straight up not a Lightner. I really think it could be something else entirely; it's just too physically weird to be a Lightner in some dark armor.

My theory: a darkener/pure darkness-ner who gained access to a Lightner's soul (maybe Kris's original soul if the soul we play as isn't)

Queen makes a point about determination (which has conspicuously not been elaborated on since, despite its importance, especially in Undertale) in chapter 2 during her speech about the knight.

Spamton NEO wanted the soul for... something; this was also never elaborated on.

Queen specifies that it's the determination itself, not "being a Lightner," that allows one to make fountains; if Spamton knew what he was doing and really could absorb our soul, that would mean darkners have already been presented to us a means to gain determination.

As for what this darkener could be, maybe Asriel's or Dess's tears?

* (There's a peculiar book here. Read it?)
* And so wept the fallen star, making rivers with its tears.
* Then, slowly, from the bitter water, something grew.
* It looked like glass.

Maybe that's what this was in reference to.

TLDR; I don't think the knight is a lightner or even a corrupted lightner; I think it's a darkner who gained access to a lightner soul, allowing them to create fountains and possibly persist in the light world.