I wanted to joke and say maybe Rudi was the knight because the body looks male. The shoulders are wider than the hips. His medical condition might be due to whatever he is doing to become the knight.
Then again, the horns look close to Carol's.
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Who knows, maybe it is an amalgamate of Carol and Rudi combining into one person to become something stronger.
I prefer it to be Rudi tbh.
The whole "it's potentially holding a bat before it's a blade" or "the knight is cocky like Dess (might be)" could still be attributed to Rudi, because that's her father and he could have taught her how to play and his attitude could have rubbed off on her. He does like to threaten to kick people's asses.
And he did stress to Kris that he wants Noelle to toughen up, and that could be part of why Kris torments Noelle so much, I mean aside from being a weirdo that likes to get a reaction out of his (maybe?) crush, he could also be trying to toughen her up.
Rudi's illness could be the consequence of going all knight mode. The knight does look weird and sick when it appears, drooling and hanging there. I think people rule out Rudi because the sickness without considering Rudi being sick BECAUSE of being the knight.
The only other reason I'd want it to be Rudi is because Dess being the voice in the coding makes it more interesting, to me, if she's simply gotten so lost that she is somewhere beyond the game boundaries, like Gaster. Noelle had a penchant for breaking games, maybe Dess did the same? But if Dess
is the knight, I think I'll be okay with that too. The knight is one of my favorites in the game, just because how fun (albeit infuriating) the fight is, and Black Knife being bumped up to an instant favorite deltatroon song. Whatever or whoever they end up being, I just hope it's not underwhelming.