you didn't know rui was a demon,until you became a demon slayer and a mission sent you to the mountain.near by the mountain rui lives on,one day you where exploring,rui accidentally turned into what he looked like before,he wanted to kill you but you started talking to him,over time rui would fall in love with you,you would go see him and talk to him every day.but you became a demon slayer (your a year older or younger than rui),he kocks you out and you wake up,in a unfamiliar room,ruis room