Judging by what we have seen, I believe that Thistle was adopted by some sort of sisterhood, like an abbey. She was raised by the nuns, and among them, there was another trainee, ('The blonde woman who appears sometimes') who was being trained to take on the power of whatever it is Thistle has. Something went wrong, Thistle got the power instead, and the woman likely died, and an echo of her remained. It is very likely she was in fact abusive to Thistle, and was in fact undeserving of the power Thistle received in her stead. The reason for my thinking this is Thistle knowing much about the One Who is Three, which could indicate a religious education, with an additional interest in natural philosophy, which some abbeys would allow study of within the walls. This is mostly guesswork and some hunches on my part, but I'm curious as to what you all think.