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

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.

It means Who a person doesnot matter. What matters is what he/she does. What function that person performs. "What we do counts and who we are follows from that

These people almost have it.

What he's really saying, and this can apply to anything, is that WHAT needs to be determined before a WHO.

He's saying WHO follows WHAT, so you need to know the WHAT before the WHO.

It's kind of like when people try to give characteristics to a god. You can't do that.. until you know WHAT it is.. because the WHAT has to come first.

The V dude wants to be known as the WHAT, which comes FIRST before the WHO. He wants to not apply the WHO and only apply what SHOULD come first only - the WHAT.

Get it? :)

"Who" is the person, the human. "What" is his meaning, is "the idea" (which is bulletproof, as we see later), his vocation. The vocation creates the human. The message here is that we should put our meaning in life before our personal wishes.

People .... the back and forth sort of speaks for itself.

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.
Evey: Well I can see that.
V. : Of course you can, I’m not questioning your powers of observation, I’m merely remarking upon the paradox of asking a masked man who he is.

(Here, V says why he made the remark... basically, when saying “who “ he’s saying “really are you serious?” So he then says the “who” is but the form ( person) function of “what” he is . He is a masked man first and a person ( who ) second . Later he further clarified what he means when she says “I can see that”(or Duhh). So he then says that he’s not questioning he powers of observation ( what she sees ... the person (who) behind the mask( what he is)., but questioning the irony (paradox) of asking who he is ... that’s why he has the mask on! Like asking a blind man “ did you see what just happened a sec ago?”

Evey: Oh, right.(oh yeahhh)
V. : But on this most auspicious ( important, special )of nights, permit me then, in lieu of the more commonplace(because it’s not a more formal place) soubriquet(very important!!...he’s going to give her his aliases or nicknames), to suggest the character of this dramatis persona(dramatic person/ over the top character). Voila! In view humble vaudevillian veteran( he’s a Veteran of the stage), cast vicariously as both victim and villain by the vicissitudes of fate. This visage, no mere veneer of vanity, is a vestige of the “vox populi” now vacant, vanished. However, this valorous visitation of a bygone vexation (Guy Fox is a dead person . He brought back)stands vivified, and has vowed to vanquish these venal and virulent .... etc...