I don't know if I want her to be half transformed, per say, but maybe the originator of the curse, so the most afflicted. Just something which sets her apart from the others in the group and could motivate her to act in this way.
Part of me is wondering if I should just excise her as a character, as she was in my original draft but may not fit now, and actually make the other "hidden" person in the house a straight up creature or non human entity. Then again, if I can just as easily swap a human character for a non human one, that could be a sign that I don't have a grasp or a place for that human character any longer.
My downside with cutting her out altogether is losing the impetus or character driven reason my love interest would hide this other character. If the culprit is someone the love interest cares about, it makes sense that she would be invested in hiding or protecting this person even if they are doing wrong. And, that wrong could be motivated-- maybe her aunt hates the town, rejects her humanity or has a toxic relationship with her niece, despite the love they also share. My lead herself is bitten and struggling between her humanity and new nature, so having a character who has fully rejected or is rejecting her human side may be a nice foil or warning for the lead. Another layer to this whollleeee thing is that the niece is gay, as is the lead, and villainizing the love interest feels wrong to me too. She and her aunt aren't villains, I think, but they are morally grey.
I do see the aunt either dying, or fully transforming and escaping off into the wilderness. I don't envision this story having a happy ending, per say. It's a gothic noir type story, so there will certainly be a resolution, but something maybe that is grime and inevitable.