Clarisse is the catalyst. She stimulates Montag's thought processes. He already had the seed of rebellion in him because he had already stolen books, but Clarisse is the one who made him think, who activated those thoughts. He enjoyed it, and he missed her when she wasn't there anymore. Bradbury removed her because the story was about Montag's thought processes, not hers. She would have gotten in the way and confused the issue. We would not have been able to determine if the thoughts were Montag's or Clarisse's.
Clarisse and Mildred are exact opposites. Clarisse is inquisitive and questions everything; Mildred is robotic and questions nothing. Clarisse experiments. Montag asks her if she goes around trying everything once, and she replies "sometimes twice." (pg 21). Mildred on the other hand is fearful. She is afraid of trying anything new and when Montag introduces her to the books, she burns them. Montag describes Clarisse as "peculiar" (pg 23) in that she doesn't fit into the mold society has designed for everyone. Mildred is "obedient" and doesn't want to go against any of the rules of society. She is willing to accept anything the society tells her. Clarisse is introspective. She thinks about everything she sees and smells. She says "Do you want to know what I do with my time. I just sit and think." (pg 23). Mildred is mesmerized by the TV. She does not think on her own, and she lives her life in a trance thinking that the people on TV are her family. Clarisse is "social" and says, "Being with people is nice" (pg 29) She finds it easy to talk with Montag even thought when he first meets her, she doesn't know him. Mildred is indifferent and unfeeling. She doesn't care if Montag is sick, insisting he go to work anyway. She doesn't care that the old woman died. "She's nothing to me."