In Fahrenheit 451 who rarely appears but plays an important role and why?

The character Clarisse McClennan does not appear in the story very much but has a dramatic impact on Guy Montag.  Clarisse is a young girl who lives in the neighborhood where the Montags live, she embodies the exact opposite of what society is trying to maintain and create, mind numbed robots, Clarisse is a vibrant, young woman with an imagination, who thinks, contemplates the questions of life and has a profound impact on Montag.

"Clarisse represents innocence. She questions the rationale of the ideas that govern Montag's life and is the stimulus that makes Montag begin to doubt what he is doing. Clarisse is shown in contrast to Montag's wife, who totally accepts the values of the society, even when it is harmful to her health."

Montag is secretly, in his own heart, privately, questioning the life that he lives, especially after the firemen burn an old woman along with her books because she refused to leave her precious books.  Montag encounters Clarisse and she asks him questions like: Are you happy?  A question that Montag has never thought about before.

It is because of Clarisse that Montag realizes the there is something essential missing from his life and his marriage.  Although Clarisse dies in a car accident, which Montag believes was murder, her influence on him is permanent.  He realizes that he must escape the life that he has lived and join forces with another character who is not seen very much in the book, Faber.

"It is Faber Montag turns to when he must flee; "he just wanted to know that there was a man like Faber in the world," he rationalizes. However, it is Montag who ends up inspiring Faber to fight back against the firemen—to "do the right thing at last."

Faber is an old English professor, who no longer teaches because there are no more schools, he is also a member of a radical group that has memorized the contents of certain books.  Faber becomes a mentor to Montag, helping him to cope with his life as a fireman and then sheltering him after he runs away from his own burning house.