Clarisse affects Montag because she makes him think about himself and his life, as well as his role as a firefighter.
When Clarisse introduces herself, she says “"I'm seventeen and I'm crazy.” She goes on to describe how her family is unusual because they like to drive slowly and sit and talk to each other. She makes Montag think, especially when she asks him if he’s happy. He has never thought to ask that before.
"Are you happy?" she said.
"Am I what?" he cried.
But she was gone-running in the moonlight. Her front door shut gently.
"Happy! Of all the nonsense."
He stopped laughing. (ch 1)
Nobody in Montag’s world is happy. People commit suicide all the time by jumping off buildings or overdosing on bills. They rarely talk to each other, and mostly just watch television in the parlor.
After Montag meets Clarisse, he starts to wonder why he is not happy, and if it is possible to be happy. He questions his profession, and his wife. His entire life changes, even though he has no more encounters with Clarisse after she is killed.
thankk you((; and um 1 more question: why does the author vanishes her from the novel