Somebody has certainly made this theory before, but maybe not since the release of 3+4.
My theory is that there are still multiple knights, just wearing the same armor, and it could just be that the armor is what gives the user the weird knight powers, and this would explain why each of the appearances of the Knight we've seen so far seem to behave differently.
The knights we seem to have seen are:
Super Serious Knight only does what they came to do. They act completely emotionless and fierce. Appearances: ch3, possibly first ch4 appearance
Overly Aurafarming Knight tries their darndest to look really freaking cool and succeeds. Appearances: partway through ch4, at the end of the First Sanctuary
Annoyingly Arrogant Knight carries out their duty, but not without being condescending and cruel towards the heroes. Appearances: at the end of ch4, summoning the Titan
He's humming about how much his car kicks *** and how he can watch Madagascar while he's driving, saying that those animals so funny they make him want to merge without looking
How did Overly Aurafarming Knight leave the upper room when there was a locked door and the windows are confirmed intact? Why would they leave? Why do AAK and OAK need to be different people? They act the same. Why do any of the knights need to be different people? The knight aurafarms a lot in chapter 3 (see: final attack, the SWOON cutscene) and they act plenty serious in chapter 4 (fountain opening scene, them knocking K&S away from the fountain). This could be explained much more easily by them being the same character.
MultiKnight theory is like arguing Queen in the beginnning of Chapter 2, meme Queen (Kris Get The Banana, etc.) and Final Boss Queen are different people.