That is professor W.D. Gaster, a royal scientist who preceded Alphys and is implied to have built the Core located in the heart of the underground. It has been theorised that Gaster is related to Sans and Papyrus, since he is a sort of skeletal being who is named after a writing font (wingdings,) as well as sharing his name with the floating skulls that Sans uses in your fight against him (Gaster Blasters.) I would not blame you if you never found Gaster no matter how much you kept searching, because he was removed from the game. Due to some freak accident involving the Core, he disappeared without anyone noticing he even existed. Bits and pieces of him are scattered across the game code, and if you keep fiddling with certain codes (such as adding capital F to the word 'fun,') you will meet some creepy NPCs talking about the professor and eventually meet the man himself. Don't expect a boss fight from him, though, since he vanishes when you go and try to talk to him.

he's kinda in the middle of something