Just Making Up Some Theories - Tumblr Posts
Papyrus does this before you fight him too, and Undyne might do this too. She concealed herself in the shadows though, which I think was part of the environment rather than an ability she could do. But Papyrus is in an open environment, no indicator of there being any overcast shadow from above him and Sans. ALSO... Lancer and Roulx Kaard can seem to... Slide around and stuff... Do we know someone who slides around as if they aren't bound by physics? Sans and Papyrus maybe?? Obviously we have seen Sans teleport, but Lancer also seemed to do this when he claimed he was going to follow us as a team, late in Deltarune ch1. Also, Papyrus may not teleport necessarily, but DOES run on the air as if he could float or... break the laws of physics?? Which both Rouxls Kaard and Lancer do. Who has a quantum physics book? Sans. BOOM. Sans is Lancer. Papyrus is Rouxls Kaard. Lol maybe not but interesting to speculate lol Honestly kind of hoping that Handplates is the true head cannon lol
Deltarune Theory
So I’ve been seeing theories about Sans and Papyrus being recently and I just found some evidence to back it up.
I realised that Darkeners seem to have the ability to shade out their bodies; for instance, Ralsei.
Ralsei’s face is shaded out through out the whole game until the end when he reveals himself. Some might say that’s just the hat doing it, but then why when his hat comes off his face is still shaded out.

I also doubt a hat could really shade out someone’s face like that where you could barely see them.
Another character who is shaded out is the King when we meet him, but then he just becomes visible all of a sudden once the battle starts.

Lancer is also shown to be able to shade out his entire body.

Now going back to Undertale Sans is shown to have the same ability to shade out his body and then become visible.

From what I know of none of the other monsters in Undertale do this except for Sans. In Deltarune it seems to happen at different times in the game. Maybe this isn’t the case but it could be another link to Sans and Papyrus being Darkeners.