201X/Rapture is a GONER, on a specific fun value, he replaces sans until a True reset is done, he truly thinks he is sans, but he is not. Everyone in the undeground knows something is wrong but they can't tell what. He is stable if you do the game normally, but you can easily make him doubt himself, which makes him extremly unstable in this case. He can go attack people or even morph into an unstable state in such moments, all to try and prove to himself he is sans, and that the others are the problem. In his stable state he acts like normal sans, nothing out of the ordinary.