Mandy was once a childhood friend of Mark's a long time ago: long before Mark was a jerkface, long before Mark was an isolated friendless boy getting picked on in school, Mark and Mandy were inseparable bffs. Mandy cannot remember this now, at least, not consciously: she has had so much happen to her that her memories are blocked out. However, during rare, lucid dreams, these memories sometimes creep back in, as through a doorway that was not fully closed...
I had some trouble thinking about how I was going to introduce Mandy's complicated character fluidly into the story: for one thing this is taking far too long and I have so many other characters that are being introduced. I also wanted to introduce the character in a way that sets up the conflict of Mandy's 'amnesia' regarding her past without immediately throwing in your face how badly traumatized she has been.