To answer this question you need to think about what the characters do - their role - in this novel, then relate their action to the wider theme of the book.
Montag could be said to represent a spark of defiance against the totalitarian regime within which he operates. The novel is basically his story of awakening and movement from unquestioning robot to active rebel. He is clearly overwhelmed by his thoughts and feelings and is unsure how to operate, often chosing his own course rather than following the advice of people such as Faber. Interestingly, some of his actions seem to be horrific, and these actions seem to be completely involuntary - he doesn't seem to know what his hands are doing. An example of this would be when he sets his supervisor on fire. This however reflects his deepest desires to rebel against the situation he finds himself in. In this book Montag engages in a desperate quest to understand and define his own life through books - a quest in which he often makes many rash decisions, however one with which we can all identify as the quest to discover the meaning within our own empty lives.
Faber is the English Professor who guides Montag in this quest and exposes him further to books. Interestingly, Faber seems to engage in a battle with Beatty, Montag's boss, for dominance of Montag. He is a complex character who often gets Montag to do the things he was to cowardly to do himself. At times he seems to encourage Montag to think for himself, but at times he appears to want to dominate him. The fact that neither Faber or Beatty can volcalise their ideology in a convincing manner perhaps reflects the limitations of ideologies - they only contain truth up to a point, and we are left to define our own lives when we reach the limitations of a particular ideology. Faber then could be said to represent a guide of Montag, or a manipulator.
Montag represents to me knowledge and human curiosity. Montag represents every man, freedom repressed, and freedom from opression. The turning point in this novel is when Montag burns Beatty along with his own home. He is symbolically burning up the oppressive force in his life. Montag's character shifts from being a good citizen to being in open rebellion to the oppressive regime. Montag is now a criminal. This brings up the question: Is there a criminal in every person?
Faber represents freedom of thought, freedom from opression, and veiled rebellion. Faber has knowledge, but he is limited in his practice of what he knows because he is living under the oppression of the current government. Faber is also a criminal, but he has managed to live in society and keep his secret.
Faber's opposite is Beatty who is also very well-read. Beatty hates knowledge, ideas and free-thought, but he is well read. This is hypocritical if you think about it. Beatty has all of this knowledge, but he's destroying it while keeping some for himself. Faber also is well read, has stores of knowledge, but is willing to share his knowledge as long as he knows that he himself will be safe.
Faber represents selflessness and humanity. Beatty represents selfishness and the status-quo. Beatty is a criminal, but he excuses his reading as part of his job.
This is a very subjective question. The answers that you come up with should be supported by quotes from your book.