Rudolf is the knight, here's why:
>Toby Cox is a big fan of japanese media and mystery genres, and he is a fan of touhou
>Going by what the stereotype says, he has most likely played Higurashi and Umineko, which is inspired by mystery novels.
>The first rule of Knox is: The culprit is always someone mentioned (AND SEEN) early in the story
>Mayor is mentioned very briefly, so they cannot be the culprit
>Dess is mentioned very briefly, they cannot be the culprit.
>Rudolf is seen and heard. The red herring is him being "sick", he is, but it's because of opening dark worlds.
>Deltarune's last prophecy is... something i won't mention. But I know it.