Judging from some of the things Gabriel has said, we know he promised that he would bring Emilie back, but the way he says some of these things, it sounds like Emilie was the one who made him promise that.
This is only speculation, but it seems plausible that Emilie owned the butterfly and peacock miraculous, as well as the book. Something happened to her that made her make Gabriel promise to use the ladybug and black cat miraculous to bring her back.
Since Gabriel can’t read the book about the miraculous (hence why he doesn’t know the recipe for the transformation potions), but knows about the equal exchange rule, according to Thomas Astruc, someone must have told. It could have been Nooroo, in an attempt to discourage Gabriel from trying to get steal the miraculous, or Emilie could have told him.
On a personal level, I think it’d be way more interesting if Emilie was evil, or at least morally grey, as the dead, perfect family member is very overused, and has a tendency to distill the character into a stereotype, who’s only motivation is being good and only role is their relationship to the people that love them. Making Emilie a bad guy could potentially be a really interesting spin on the trope.
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