Lab coat guy! He's sans with a balloon on his head posing as a CORE technician so he can go CORE fishing. (Don't ask.) He talks with sans' speech byte and in comic sans, so it's easy to see who he really is. His mouth doesn't move, since it's crudely drawn on a balloon.
Click him to see the image which I took Lab Coat Guy from as inspiration (It's from a spectrogram of Mus_smile, so is this technically Gaster fanart that has nothing to do with Gaster?) If Lab Coat Guy becomes the next Pluey, I MADE HIM.