Rudy as the Knight makes the most thematic sense and I will argue it until the day I'm proven correct.
Imagine you've lost your daughter, your best friend is going nuts in a divorce, your other daughter is fucked up in ways you can't explain, and the closet things you have to sons are either killing themselves or gone away.
Think about it, the Queen is how Rudy sees Carol, as a good overbearing mother. All the dark fountains, except the ONE made by Kris are within walking distance of the hospital and have closets to hide in.
What would you do if your daughter was killed (Toriel actually hit Dess while drunk driving theory), but you found a way to bring her back to life in the Dark World? You would want to spread the darkness just for a chance to have her back, but when she's outside the bunker she turns to stone.
Rudy also has a unique face sprite that only important characters have, he has multiple unique sounds (The knight also laughs with the same pose almost), and has been shown to get out of bed at this point.
If he can retain knight form somehow in the light world (making himself sick) he could easily have carried Undyne.
Deltarune has themes of escapism, and who wants to escape more than a parent who lost their child and is dying?