Who do y'all think is the knight? 

All clues aim towards Dess (Noelle's lost sister)... but seems a plot-twist too obvious for toby's writing.

As much as the Dess the Knight theory makes sense,
I still bet it's Carol. If you think about it:

(SPOILER so read at own risk)

1. Kris was calling someone (who may be the Knight) while drinking a cup full of chocolate milk

2. Said that Susie mustn't get guitar and the code

3. After the fuss between Kris and us, Kris calls the unknown number again (who may be the Knight) and tells them about Susie who is now holding the guitar

4. The unknown number tells them that they'll be coming right there (to the Holiday's mansion) shortly

5. Later, Carol comes back home quick and just takes the guitar right off Susie's hands

Which makes me think that's suspicious because in what world an "innocent" person just pops up in time where a character has the most significant plot mechanism and immediately take it away?

More other reasons:

1. There are unused dialogues of a character seeking for help because they're lost and in the dark. This may be Dess in my opinion

2. Dess' theme (Lost girl and findher.ogg which is heard in the Spamton Sweepstakes) titles of Dess being the lost girl, which hints that she is trapped.

I have more to reason but they may be foolish to think. just message me if you'd like to hear more about my theory!!!