If the Soulless route is started you'll see it at the start as Unknown will ambush the person trying to knock on the front door.
https://mega.nz/file/KdkiiSBT#_rjQUv6QP3uYBddeK6XW4dumDoviQwIqGKBZiZAE2j8 here is a link to a guide, i dont know how much more ways I can try make it clear, encounter unknown, go through the sequence until she asks for your soui, give it to her, re-open the game and load the save file for the fun you was in when you triggered her, complete an ending and you should see Unknown in the post credits. If she appears then it's time to make a new game and watch the fun unfold.