So, another re-incarnator or something? Seems like she may have travelled to the past, instead from another world. Not sure I want to reread this right now to sort things out, though.
She might not be, considering she was communicating with her old man and apparently following his instructions. It could be that she's just a regular kid, seeing how she even needs to use some drug to change her eye colour, and is acting based on "common" knowledge. Not common in the sense everybody would know it, but not also so esoteric you would necessarily need to be a reincarnator. Even the plan seemed to be to make the girl a priestess so that she could systematically go through orphanages to locate Damian, which suggests they were no wiser than anyone else.
That being said, I don't quite get what the core of her plan is, how is she supposed to get close to the duke and why does it require green eyes. Was this priestess girl's aim to take Ellie's place? However, wasn't Ellie, the daughter of the Empire's famous cat burglar, close to Damian only by coincidence? Well, who knows, maybe Ellie's mother was famous beyond being a thief. Or maybe she wasn't a thief in the first place, but that was only a fabricated accusation to get rid of her for other reasons. I seem to recall someone suggested that already.