What does `who is but the form following the function of what` mean?

Can someone please break down that sentence of Alan Moore for me? I can understand that it somehow means that who has the same function as what or something like that but I don't understand how that structure means that, I would also appreciate other examples of such structure. I also understand something like I'm but a very brilliant guy to mean that I am nothing but a brilliant guy.

This is from the movie V for vendetta where Evey asks V(it's a name of a man who always wears a mask) about who he is when they first meet.

Evey: Who are you? V. : Who? Who is but the form following the function of what and what I am is a man in a mask.

It is just a cryptic/clever/verbose way of saying that "who are you" just means "what are you" ("who" is just another word for / has the same function as "what"). To which he responds that he is just a man in mask.

Form follows function is a term from 20th century modernist architecture, and has influenced many aspects of modern design.