Point 1: King has potentially seen and talked to the Knight. If Kris was the Knight, King would've recognized them, he ain't dumb. Point 2: Kris moves sloppily without the SOUL, to the point at which they can barely get over their own room, so how would they even be able to get across town, completely undetected, break into the library, open a Fountain, get back home UNDETECTED, potentially climbing up their own house up to their window in order to not wake up their mother, put back their SOUL and go back to sleep in such a state? Point 3: Why would Berdly and Noelle jump into an endless abyss of darkness? Common sense indicates they wouldn't (yeah, I know Kris and Susie enter the closet, but that's more of an act of recklessness and stupidity). Point 4: The fact that they opened a fountain does not automatically make them the Knight, since as Queen said, anyone can open a Dark Fountain. If we use the capability of creating Dark Fountains as vector to make Kris the Knight, then we might as well make Berdly the Knight too (before you say anything, if Berdly couldn't open a Fountain, then riddle me this: why was Ralsei so pure and utterly scared? If Berdly hadn't the ability of opening a Fountain, then SURELY Ralsei wouldn't have gotten so scared. Right?).

Of course, this is ignoring the theory that the Knight is more of a title than a defined, singular, entity. If that theory ends up being true, I said nothin'.