Mildred and Clarisse pull Montag in separate directions. Mildred represents the end result of ambivalence: she doesn't have opinions of her own, her "world" is her entertainment, and the last thing she wants to do is stand on a principle. Clarisse, on the other hand, illustrates how philosophers and thinkers end up isolated, but satisfied and willing to defend what they believe.
Montag feels a sense of obligation to Mildred because she is his wife, but he sees the shallowness of her life. He wants more from the relationship, but Mildred is more in touch with her virtual family and her girlfriends. Her primary interest in Montag is the income he generates so she can get her fourth wall.
Montag is intrigued by Clarisse. She notices the small things and likes to listen and think and ponder why people do the things they do and why society moves so fast that no one notices anything anymore.
As the story progresses, Montag begins to see that Clarisse had the right path. Unfortunately, society doesn't tolerate individualism well. This is one of the messages Bradbury sends through this book: to be different is to be rejected, but the satisfaction of being unique makes life fulfilling and worthwhile.