(Up above is the first version. Then I discovered a very useful tool I should've found earlier, but oh well...
So down below is the evolution!!!! UF Sans evolved to Phase 1 UF Sans!!! [That's how it works in pokémon, right?])

As the title says, this is UnderFell Sans...
At least how I imagine him...
LOL

I mean, surely you would agree with me, right?

This is obviously Fell/Red and you can fight me if you disagree.

Don't steal da art or reupload without my permission!!! I didn't watermark it... don't take advantage please!

Til next time lol