Well Imp, you might just warm up to my favorite female character just as much as to Varys
First of all, Mellisandre is fire embodied, GRRM went to length giving her appearance reminiscent of the element she wields. But as fire flickers, burns stronger, or dies to the cinder, so does her heart (and her hart is the master of her self, and that is why she is my favored character), and Mel is torn between what she sees in fire, and hers uncertainty of it's meaning.
Her past is still clouded in the books, was she a slave girl named Melony, sold as part of the Lot Seven? Don't know, too many guesses possible relying only on short excerpt you quoted. Will possibly be revealed in the future.
Needle and you gave some good guesses, and we will see how her story goes, but here is something I've been thinking: Jon is slowly turning into Ice embodied, while Mel is (like I mentioned) fashioned in image of Fire. What if it is not A Song of Jon and Dany, but A Song of Jon and Mel? Could it be that this time AA is not Lord of fire AVENGER, but a CONCILIATOR between two opposing elements, as Needle suggests? This appeals to my imagination, as it gives many unsuspected possibilities, and negates most of the valid theories in this forum.
Why would she hear whispers of the Great Other, is she so important, or just extra sensitive? And if so, her role in the future might be much greater than up until now (wishful thinking).