Seriously, I have such ideas for this concept. My original idea was a Priestess of Bhaal (because I got excited when I came up with the idea and made a portrait... yes, I am a dork), but it could be either.
Bhaal is dead of course, but dead gods can have dead power through certain artifacts and such. The idea would be that one of her ancestors was a priest/priestess of Bhaal, the head of one of his temples even, and handed down a powerful relic that contains some of Bhaal's power. This could simply be a ring or an amulet that can't be removed from the character, and it would allow the priest their divine power.
I think this would be a really intriguing idea to explore. I imagine her odd, not quite to the point of insanity, although people would hesitate to call her "mentally healthy" (much like Minsc!). Since she essentially WORSHIPS BHAAL, she would of course be absolutely taken with the PC (and later, Imoen). It would be a really cool, unique relationship, probably with a romance option. In both cases, the PC would understandably have a HUGE effect on her... and depending on how they treat her, maybe she could even either a) become more normal, less fanatical, or even renounce Bhaal, or b) become a complete tool for the PC.
If anyone is interested in hearing more, let me know.