I have a feeling that Joans ex girlfriend will make an appearance

She already made an appearance. It's Lakam.

wha-

Lol. Sorry, let me explain.
I have a feeling that Lakam was human before she became a demon, she may have been Joan's ex-girlfriend in high school and somehow died and became a demon. The reason why Joan wasn't able to recognise her was probably because of her hot demon look. As for her sister...she may not be her real sister? These are just my two cents though...