The truth is V's identity, though multi-layered, is clearly established from the beginning of the novel. Save for Prothero's interrogation and Evey's imprisonment, V only appears in the guise of Guy Fawkes, the 16th century radical who was willing to go to extreme lengths for his beliefs. This description certainly fits V. Like Fawkes, V has unwavering conviction in his beliefs, and is willing to go to every length to see that the current rule is abolished. In this respect, V and Fawkes are revolutionary kinsmen. But it is a mistake to simply see V as a literal modern-day Fawkes. It's not Guy Fawkes himself that V wishes to be, but what Fawkes represents: rebellion. Strip away the context of Fawkes and his conspirators and what you have is rebellion at its most pure form: rebellion against government, against religion, against ideology.
But V isn't just about rebellion. He may look like Fawkes, but he is far greater than him. To understand that, take a look at V's name. "V". It's not even a name at all--it's a letter. Its meaning and purpose changes depending on the context it's used in; it is, in every sense of the phrase, a place to start. Moore reminds us of this fact by the ever-present wordplay found in his novel. Even the novel's title, V for Vendetta is a play on words; it's a twist on the famous World War II slogan, "V for victory". Every chapter title in the novel also begins with the letter "V".
The end result is the most obvious: V stands for many things. (V for "variable".) When V first introduced himself to Evey, he calls himself a villain (13). (V for "villain".) The description is appropriate, as V stands for the polar opposite of everything Norsefire represents. He is their enemy, and he knows this is his role, as Guy Fawkes was the villain in the eyes of King James and Protestant Church.
V is also for "five". At Larkhill, the concentration camp where V achieved his physical and psychological metamorphosis, he was kept in the fifth room, labeled with the roman numeral for five--"V". Adopting that symbol as his name, V pays respect to the forces that made him who he is. But on a larger scale, the V is now symbolic of all the people who were imprisoned and tortured by oppressors. (V for "victim".) By using his room number, V is a living symbol of all the casualties of Norsefire's genocide. Then there is Valerie's letter. ("V" for "Valerie".) Arguably the most important part of V's persona, it is this letter that transforms the person V was into V. "But it was my integrity that was important. Is that so selfish? . . . It's the very last inch of us, but within that inch we are free" (156). In a very real way, he becomes the physical embodiment of that one inch Valerie talks about. Valerie says "we must never lose it, or sell it, or give it away. We must never let them take it from us" (160). Beyond V's rebellious aims, he also reminds the populace about their identity, their integrity, that last inch of being that they had forgotten about: the freedom to be themselves, despite whatever anyone else tells them to do or be.
(To digress just a moment: there have been many people who think that V is in fact Valerie. This has always struck me as a disservice to the story. If Valerie were V, that would significantly diminish the emotional impact of her letter, not to mention make V extremely egotistical for creating a shrine dedicated to herself. Futhermore, both Prothero and Lilliman refer to V in the masculine once they realize who he is, and that alone should settle the question of V's gender.)
It's as if V, the person, becomes more than this menagerie of ideas. He becomes a force. He becomes an instrument through which he takes all these ideas and puts them into action, extracting his vengeance, pursuing his vendettas, preaching his values, until his vision is vindicated.
This is why Evey becomes V at the end of the story. (V for "Evey".) As an idea, as a force, the original V can only do so much. He can rebel against The System, he can awaken the value of Valerie's One Inch to the populace, but once that is achieved, he no longer has a purpose. So Evey steps in, picking up where V left off. She continues the cycle--as V adopted the guise of Guy Fawkes, Evey adopts the guise of V, continuing his spirit while becoming something more than he could ever have been. "I will not lead them. But I'll help them build. Help them create where I'll not help them kill. The age of killers is no more" (260). The first V was a killer, a destroyer, in both the figurative and literal sense. But the next V, Evey, will be a teacher, a builder.
The question of V's identity is a tantalizing one, but it is irrelevant. Evey understands this. She says: "If I take off that mask, something will go away forever, be diminished, because whoever you are isn't as big as the idea of you" (250). She is telling this to herself, but to the reader as well. What really matters isn't who V is, but what he has come to mean.