Clarisse had a huge impact on Montag's life.  She made him realize that he was not happy.  Everything he does in his everyday life is routine.  He even realizes the coldness between him and Mildred.  Clarisse plays the role of the rebellious person.  She refuses to be unhappy and not talk like everyone else in this particular town.  She lets Montag know that he is not in love with his wife by rubbing the dandelion under his chin.  She also asked Montag why he chose the career he did.  Clarisse is generally concerned about Montag's happiness.  She vanishes from the novel because, in my opinion, she wants to see if Montag will change on his own.