That's... actually a hard answer. You are right that Sora is based heavily on Itsuki, but also that she's imported from SRWUGA. That game didn't let us call them AUs, though, so they got a whole OC canon we played start to finish, which is briefly covered in her app here, and I can expand on it. I called her an OC here because, as far as UGA was concerned, that's what she is, and so it's what I default to calling her.

I am unsure how to proceed... I am fine calling her an AU of Itsuki (which she is), or I can keep her an OC. Which would you prefer?

EDIT - I am 99% sure the right answer is to just call her an AU of Itsuki and be done with it... that's not going to mean I have to rewrite my app, is it? If it does I'm fine with that, I'm just hoping I don't have to.