Literally everyone that hasn't been in a dark world
Noelle
Tenna
Gerson
Mike
Friend
Everyman
The Forgotten Man
Probably some others that I don't have the patience to list here
Tenna dying:
But he was never on the same frame with the knight
I think we can cross off Everyman (gag?), Noelle (a party member, also probably not from a tv tropes pov), Gerson (helps kill a Titan), Mike (likely not considered), FRIEND (mostly hidden character, only there to creep nosy people out), Tenna (tv tropes once again), idk if Toby would even think about using tfm.
Candidates could be:
Carol (I feel like this is plausible, but I'll try to remain as neutral as possible.)
Papyrus (Many believe in this, pointing to evidence such as body shape, however deniers point to Papyrus's personality, so...{And also Kris works with the Knight, IDK if they even know Papyrus, as he's apparently new.})
Amalgate (Self-explanatory, not much to it.)
W.D Gaster (Likely not.)
Another Lightner not yet considered who has not yet been sighted within the Dark world(s) (Perhaps. Perhaps...)
I believe in GasterDessKnight bc of the Another You theory (peak theory btw)
Oh right
Dess (Missing, so perhaps? Also, could be in contact with Kris, that would make sense, but why, if they can go around town, have they not returned to their family? Or did they run away from home? How very, very interesting...)
W.D Gaster (Probably not, I'd say, because he's not meant to be revealed to most people, and the KNIGHT is a major character. I just think that Toby wouldn't put him onstage for such a long time...)
Your forgor Asriel Knight (which... not the best theory but it exists)
but I didn't
Talking to Palladium
oh
Asriel (Some suspect Asriel, who is supposed to be studying at college, and the college would likely inform Toriel if he were away-however, Toriel does get imprisoned in a dark world roughly the same time the KNIGHT appears, so...)
He wanted to kidnap toriel so he wouldn't be grounded
Surely there's better ways to avoid being grounded than that, right?
The twisted mind of the teenager.
Nvm I stupid knight is a lightner
And what is Asriel?
Finally, Knight Asriel theory getting more recognition
I'm not an Asriel-Knight follower, but I do vouch that the theory exists
NUBERT KNI-lol