Are you OK with Changlelings? I'd be looking at using the Ash Hag parentage trait from the recent Cheliax, the Infernal Empire book.
She'd be a Shaman using the Flame Spirit.
She'd be an Isgeri native whose hag mother used to large number of orphans of the Goblinblood Wars to hide her daughter in. She did not count on the child being raised by the Sisters of Golden Erinyes and the discipline that would be drilled into her.
While the sisters did not convert the child to Asmodeus and she had no skill for their brand of martial art they did teach her the mental fortitude associated with it. The sole reason the child did not convert to Asmodeus is that she has had other beings speaking to her since her early childhood. Beings of warmth, light, purity and remorseless destruction. To her Asmodeus is nothing but a powerful fire spirit who has many lesser fire spirits in his employ. As such she appreciates him and honors him but does not actually worship him and he's not the only one for she honors all fire spirits, big and small (basically all deities with the Fire domain are included in her prayers as well as many non-divine beings of fire).
She would have come to Cheliax to get away from her mother's Calling. Not because she feels attracted by it, the nuns' teachings and her fiery companions make resisting the Call relatively easy, but because it's an annoyance that distracts her.
She would be lawful evil, being very disciplined but also cruel and somewhat sadistic. She'd delight in setting things on fire, preferably things she never set alight before, and watch how they burn and what the fire does to them. She is however smart enough to know that going around burning everything is stupid and as such she carefully selects her targets or even works as an arsonist for hire. She'd frequent temples and shrines to Asmodeus and other devils simply because they feel like home to her.