His skinny body could fit through the armor, couldn't it?
He wants to cause the roaring so that he gets the limbs of his Dark World counterpart.
From what I’ve heard, Kris tries to imagine the Knight without their helmet and the face is Jockington.
We, as players, wouldn't believe Jockington is the Knight. So we would not believe Kris is revealing the truth.
He grows a beard, which would only be important if he's the Knight. We would see a beard on the Knight and know it's Jockington.
The antlers? Helmet.
Kris is not working with the Knight, the person calling him was indeed Carol. Carol doesn't want Susie to learn about Dess’ disappearence, which could be for any reason itself. I can't explain why she wouldn't want Susie knowing, maybe she killed her daughter and doesn't want anyone to know? Alphys could be really sleepy at the time and think she sees Carol. Alternatively, Carol actually did kidnap Undyne and doesn't want Susie to find the code and let the hostage go. Another alternative is that she works with the Knight and kidnaps Undyne to find out the rest of the code.
Why didn't Jockington bring Undyne there if he's the Knight? Assuming Carol is working with the Knight, she probably told him to go home and let her do the rest.
This theory does tend to crack at points regarding the Holidays, but perhaps Jockington has antlers on his helmet to honor the Holidays since he's working with them.
