Papyrus: Uh... Sans. Who is this and why do they look so much like you? Sans: Uh... I have know idea what that is. Please don't say it's another one of those alternate universe things. Papyrus: Uh... what? Sans: Oh, I know what it is! It may seem to look exactly like me, but if you look closely, you can see its face is most certainly that of a Dittos. Papyrus: What is a Ditto? Sans: Well, *pulls out pokedex* a Ditto is a normal type Pokemon that has the ability to take the appearance of anything it sees, as well as copy its abilities. You can make the rest.
There is a Sans-Ditto sprite I made inside the project. You may use it if you like (just make sure to give credit of course). Credits to FreddyRemixes for "Royal" Guidance
