I had considered Body, but then her concern about MC not being pure/balanced enough due to her own modifications doesn't make sense. No, I think it has to be one of the other powers. Authority doesn't really make too much sense. Order would probably work, but also it is exceptionally improbable. Of the remaining powers, Truth makes the most sense to me.
As for the age part, it's more of an ancillary argument. In theory, the age is actually irrelevant, so long as she is younger than the MC. She could've either been removed from him as an infant, child, or slightly younger, and I don't think it would change all that much.
As for the powers, I can see Authority, but Truth is not a bad fit by any stretch of the imagination.
EDIT:
To be fair, I did call it a crackpot theory