Montag meets Faber before we do. Montag and Faber, previously, had sat together on a park bench and had a conversation that had planted a seed within Montag. He never quite forgot that conversation; it sat within his mind, and when he meets Clarisse, and starts to doubt his society, Faber's memory pops up again. So, Faber was a part of Montag's development even before the story begins.
As Montag develops, Faber becomes crucial in helping him to clarify his thoughts, and in giving him direction. Faber is the guiding force of Montag's rebellion; Clarisse was the spark that lit the fire, and Faber helped to direct that fire in a more managable and healthy direction. As Montag strives to read books, and to understand what their meaning is, he is confused, muddled and frustrated. He senses that they are important, but can't figure out how. At one point he is so frustrated while trying to read on the subway that on a whim, he drops by Faber's house, and pleads with him,
"Nobody listens anymore...I just want someone to hear what I have to say...and I want you to teach me to understand what I read."
So, Faber teaches him about the value of books, that they have quality, leisure, and prompt action. And, together, they clarify action. They come up with a plan to undermine the current firefighter's system, and Montag, for the first time in a while, feels like he has energy, purpose, and happiness to his life. Faber's voice from then on is in his ear, through the seashell, and Faber helps to tame Montag's thoughts, guide him in the right way, and interpret life as it comes at him.
Overall, Faber plays the role of mentor, strengthener, clarifier, guide, friend and conscience. He is crucial to Montag's development. I hope that those thoughts helped a bit; good luck!