Answer:
This is from the term, "frankly Speaking, I...." a directive that you are going to speak from the heart and not mince any words. No matter how you may feel after I say it, I will say it in a plain language (layman's terms) that you can understand it.
To be "frank" just means to speak frankly, nothing more. There is no "frank", because all proper nouns are capitalized, and the question here is "...,who is frank?". Since the who is a reference to person (who), and the subject is frank (as in frankly speaking).
So, in the question of "...To be frank, who is frank?, the answer is, "the one, who is, speaking frankly."