Board Thread:General Discussion/@comment-27907368-20160412010659/@comment-26887879-20160419074753

You can't kill ghosts unless they become fully corporeal with body they posessed. During Napstablook's fight if you keep hitting them, they eventually reveal they just lowered their HP on purpose. Mad Dummy is also a ghost posessing a dummy, you can only kill them in Genocide route when the shock of emotion makes them fully corporeal with a dummy.

Napstablook is only monster whose soul isn't absorbed buy Asriel during the True Pacifist run.