Demon forms are designed to terrify. To give the aggravator one last chance to back off, or (more commonly) strangle their will to fight.
Vaggie finds that her girlfriend's doesn't though. When Charlie's eyes flame, fangs grow, hair whirls and ebony horns sprout, and she radiates a different sort of intensity...
well it's kind of a turn-on if she's honest-
...but when they were just new acquaintances, and the usual carful of pendejos had stopped to hoot and jeer, Charlie had instantly stepped in front the moth and filled the air with rainbow fire. She loomed and snarled until the harassers fled.
Then she turned to Vaggie, with scarlet eyes and vampiric face holding nothing but actual honest concern and empathy, and for the first time that she could remember, Vaggie felt that someone was truly on her side, willing to fight for her.
Charlie transforms, and Vaggie feels safe.