Deltarune - Theory

Here's some other interesting stuff. This one relating to people thinking Sans is a Darkener.
Darkeners may potentially have blood, since Lancer seems to think he can turn you into a bloody mess, and he bleeds when you kill him. Susie probably can't, but Ralsei never corrects the statement since he's a darkerner.
Portraits from Dark world are colored, and Sans is conveniently black and white on his head.
Monsters in the dark world can be defeated by either sparing, or making them tired, and you need to force Sans into being tired to defeat him.
He can manipulate magic, and your very soul.

the portrait of Susie by the end of the chapter doesn't really look like Mad Dummy
At the very least she has a scar on the same eye, but with it she looks more like David Bowie then Mad Dummy