Clarisse's main role in the novel is to function in the role of innocence, both in mind and in action. She is such an anomaly to Montag in her mannerisms and her way of asking him questions that no one else will ask that she does, as the previous post pointed out, help break him out of his reverie and begin to question the society around him and its effects.
The fact that her entire family seems to be so different than the rest of the world in which they live is also important. Her family likes to sit around and talk to each other and Montag wonders why he doesn't do that with his "family." All of these play an important role in waking him up.
One of Clarisse's major roles in the novel is to get Montag to start thinking and examining his life and his world. She asks him, "Are you happy?" Despite Montag's automatic reply of "Of course," it is evident that he is not. He begins to wonder about the relationship he has with his wife and he begins to wonder about other people in a way he had not done before. He soon realizes that he is very unhappy, especially with the disconnected ways of people. No one seems to care for anyone else. Her effect on Montag then is that she is the catalyst for his emergence as a human being. Another function she has in the novel is to tell the reader about the changes that have occurred. We learn about the world as it is in the time of the story in large part through her dialogue with Montag. She explains about how fast people go, how preoccupied with sports people are, how indifferent to each other people - especially teens - are to others and so on.