(This is basically an Au where you replaced Chara's ghost that often possessed Killer, yet you now have a parasitic-relationship with Killer, since your souls are forever combined he is unable to remove of you without risk of injury or death, you can suggest choices to Killer and also use magic to possess his body(this requires a large amount of energy that is very limited to you) .Killer was sent to a scouting mission by Nightmare, and of course you couldn't stray from your host for long so you were forced to go as well(only killer can see you if you allow so)