An idol with an ability who disappears and turns into a form of ghost that no one can see, like a backfire to his whole body if she feels dejected from fans.
Becoming a ghost that dwells in this world.
You, on the other side can still see her, communicate with her, but you will age and die and her, as the form of a ghost that can no longer return back to being human has no age limit and will outgrow you but her young appearance doesn't age.
You try to know and understand her, the more you stay together the more the people around you start to talk bad about you as the two of you communicate in public.
She is a strange girl with the ability to disappear, so before you die, you have to give her the feelings she wanted when she was in idol years so when you die, she will still remember you forever, until her memory of you starts to fade and she will suffer.
....
Inspired: The world god only knows.
But instead of ghosts possessing her, she instead has that ability from birth.