Ruby in some ways respected the choice, Margareta didn't want to live in her world and she was fine with that, but respectful of it, and if she was honest with herself, grateful. What would she do with her here, how would Sam react to her, did Castiel like her more, would he grow to admire the innocent pouty peasent girl who bowed to him and called him her salvation.
But the girl was dying and Ruby would stay till that. Because Castiel wanted her to, because it seemed logical that she do, because yes, it was a part of her past, one she rarely had thought about but here it was.
"If you want me to stay, if you want to ask me anything then do? And then I'll leave you with him and your faith"
"Do you have it?" asked Margareta softly "Do you have faith in anything?"
Castiel had asked her this question before, asked her was her faith in Lucifer and she had outright lied
"In the people I care about" she replied, that wasn't entirely a lie "Demons aren't all the same Margareta, we're not all like Tatiana. Not all of us want the same thing, and me, I help Sam, I help him deal with whats inside him, what another demon did to him, that can't be a bad thing can it. And then there's Castiel. Would he really let himself love a monster?"
That wasn't a lie either, she wasn't a monster, she was the best hell had to offer, better than any of the other demons like her, she was the most loyal out of all of them, even stuck in the crappy dimension she was, and that had to mean something.