She is dead and really was a demon. You were possessed by this demon and maybe still are. When you're possessed by a demon, you think or dream about them because they reside in your mind. They live in your mind because they are attracted by things that we perceive as being negative: things like fear, emotional distress, guilt, greed, ignorance, bad habits, and even bad memories. You should try to summon her and ask her why she is there. This can give you an idea of what is about your mind that attracts her. You can also get an idea why she is there by paying attention to anything negative going through your mind during the day. If you get rid of the negativity to the point where your mind is no longer desirable to her, she will leave your mind in search of a more negative one.