Guy Montag meets Clarisse within the first few pages of Fahrenheit 451. Montag is leaving work very late at night (approximately midnight), and as he walks home, he contemplates the odd feeling that he has had that someone had been on the same sidewalk as him a mere "moment before making the turn." It's a feeling that's afflicted him for the past several nights, and yet he can't make sense of it, no matter how hard he tries.
On this fateful night, however, Montag turns the corner to see a girl who seems to be propelled forward by the wind and leaves rather than moving of her own accord. She is described as having a slender and "milk-white" face with a "gentle hunger" and an expression of "pale surprise."
When Montag asks this girl if she is the new neighbor, she very oddly replies that Montag must be "the fireman," commenting that she'd "have known it with [her] eyes shut." They chit-chat about the smell of kerosene until finally she introduces herself as Clarisse McClellan.