The McClellan family, and especially Clarisse, serve as a foil for Guy, his family, and his coworkers. Since the novel is mainly written from Guy's point of view, the way Clarisse's family is described indicates what Guy finds unusual about them, and therefore what the norm is for "normal" citizens of this dystopia. For instance, in Part 1, the McClellans are described like this:
Laughter blew across the moon-colored lawn from the house of Clarisse and her father and mother and the uncle who smiled so quietly and so earnestly. Above all, their laughter was relaxed and hearty and not forced in any way, coming from the house that was so brightly lit this late at night while all the other houses were kept to themselves in darkness. Montag heard the voices talking, talking, talking, giving, talking, weaving, reweaving their hypnotic web.