So we can all agree that Gerson knows ball, and if he does in fact know ball, then we should listen to him, right?

Well, Gerson raises the question of the Knight's motivation to the player, and this is the first time that we actually are told to think about it. 

This wouldn't just have been brought up if we weren't able to actually figure it out, or at least have a solid guess. If we just had no information that could lead us to the answer, it wouldn't be framed like an epiphany to Susie.

In fact, the very reason he says this line is to nudge Susie in the right direction, because she does not really have a clue at to who the knight is other than a lightner.

In other words, Susie is supposed to be the Watson of the scene, and Gerson is the Sherlock.

You might be wondering: 'why does this matter?'. 

It matters because the knight's motivation is a big part of who it is under the helmet, and we are supposed to use its motivation to find who it is. Susie, who is essentially representative of the least informed player, is supposed to reach the truth through looking at the motivation first, according to her mentor figure.

  
So, with all that said, what is the Knight's motivation?

Well, the motivation and character growth of basically every character in the game has one catalyst in common.

Dess.

So presumably, if we are supposed to figure out the knight's motivation, that would be a good choice. She is *the* big mystery that haunts the narrative, so whoever the knight is should be able to tie up that mystery.

This idea really only narrows it down to Rudy, Carol, Asriel, and Noelle.

Why not Dess?

Because for Dess to have come back from being disappeared before the events of the game even started would be extremely cheap. We would have had her being gone be set up the *entire* game, just for her to have been found before we even started the game? That just doesn't seem right. Especially if it was, "She's actually lost in her *mind*, not *actually* lost, and you have to pull her back!". Not to mention, her motivation no matter what type of Dess Knight theory you believe, is basically nonexistent. 

If she is fully in control we have no clue as to what her motivation could be and doesn't explain why she wouldn't reveal herself to her family.

If she is being controlled then we have no clue as to what her motivation could be.

If she has just been turned into some weird creature and wants to be turned to normal, then it doesn't really connect to the actual plot, and doesn't explain why she wouldn't reveal herself to her family.

Why not a darkner?

Because the only darkner that would actually care about Dess to have her as a motivation would be one patterned off someone else, and at that point why have the knight be a darkner? Also the fact that it exists in the light world.

  
TL;DR Because of this line from Gerson, we can deduce that the knight's motivation is a big clue as to its identity, and the motivation is probably supposed to be plausibly guessed already.

Yes, the Knight's motivation IS important, and we are probably supposed to be able to figure it out. (And so the Knight isn't Dess)