Jevil was the court jester until he went insane after meeting Gaster. He sees the world as a game, but he doesn’t want the game to end. In exchange for help with his sanity he helps the other Darkners and makes Seam happy again.

He is the only character in the game with voice acting and its unknown who it is it could be Toby fox himself or someone else: https://m.youtube.com/watch?v=MltxU4hS93Q