Ness

Ness is a human from Earthbound, He appears as a one-off character in THIS video.

while Ness himself is only there in one part of the video, What he is saying is enough to help us understand him more, Such as "WTF is this?!" implying he doesn't know Sans' bones.

He also appears fighting Dr. Andonuts in the beginning of a video Anime Chara and Sans watch for proof, Ness is not clearly visible, But the theme of Dr. Andonuts sounds similar to Sans' theme, Megalovania.

He also appears as a fusion of himself and Sans. Ness has some similarities to Sans, as explained by Sans, the Gaster Blaster works similarly to PK Thunder, but without the actual thunder.