Ao no Exorcist

Fem!Rin

Rin gets 'captured' by some demons and dragged into Gehenna (check spelling), where it turns out her dad, Satan isn't actually that bad (at least to his 'loved' ones). She gets treated like the princess she is, and though she doesn't agree nor like it at first she comes to appreciate the 'freedom' this turn of events grants her. Well, that is until she comes of age, and Satan starts trying to set her up with some 'noble' demon lord's offspring.

And so with every meeting that is set and that she runs away from, she finds love with a 'common' demon, a guard perhaps that was assigned to watch her and became her friend?

She is more or less forced to wear dresses her befitting of her status and gender (though there is no discrimination against female demons, they just lack in numbers, and subsequently get set up a lot so that the population will keep growing. Maybe Rin's womb has more potential?), and most of the time they are long and white dresses.

She does though refuse to wear what she calls 'death traps', heels or any makeup.

Rin is about 15 when the series begins, so maybe when she turns 16 is when she comes of age? Maybe time is also different in Gehenna, so is actually there 'longer' then she is 'missing' from Asieh (check spelling), and because she is half demon she ages till maybe 16 or 20 something like a normal human, but then stays that way for a long while, before growing any older.

Oh! Maybe the demons have a ritual to turn someone into a demon? And the populace demand that their princess go through it, and though she doesnt want to, someone wisks her away and forces her through it.

Maybe this is where the guard comes into the story? Like her father and brothers go to get her back once they realize she is gone (cause she cant leave gehenna so she must be somewhere), and she is then assigned a guard/protecter.

But either way she has to come to terms with losing her humanity (though her demon traits dont change much, maybe some horns are thrown in, but there like little ram horns or something, and her pointy teeth become more prominent, and some small bat/devil wings, [not large enough for her to really fly with, but she's still young so they might grow along with her horns] but that is about it. Maybe she will age some? She would definitely grow in strength [she still has her sword]. Hmm...her hair would grow longer, her waist? Below her butt? And i guess her figure would mature.)

Maybe even Mephisto takes her to Gehenna, because the church (they're the ones in charge right?) is tired of waiting and of the risk she poses, so they plan to kill her, and mephisto catches wind of this and decides to take matters into his own hands and stages a kidnapping (he probably pulls yukio into it). Because though she wont agree it will keep her alive longer.

So a demon comes through when it is just yukio, rin and her cat (kuro), yukio is 'defeated' rin knocked out and the demon then grabs her sword her cat and her and disappears into gehenna just as backup arrives.