Christy is Ben's rock. Oh, and his wife. Her strength and independence allow her to help Ben when he gets too caught up in The Wave, threatening both of their careers as teachers. And the strong relationship they have helps Ben get through a very dicey time in his life. Christy is able to tell her husband what she thinks and he takes what she says very seriously.
But beyond that, why does she matter to The Wave? Well, we're glad we asked:
(1) She offers us background information on Ben. Through Christy, we see that Ben has a tendency to get caught way too caught up in whatever project he's working on. This helps keep him sympathetic: he doesn't get caught up in The Wave because he's trying to hurt his students, but just because he has a tendency to take things too far.
(2) Like Laurie, Christy is an example of a strong woman who is not afraid to speak her mind, especially when she sees a problem. Her strength helps persuade Ben to stop The Wave before something really, really ugly goes down.
(3) Christy helps us see how The Wave looks to other teachers. Basically, she helps readers understand the seriousness of the situation: she and Ben could both lose their jobs and possibly even their careers as teachers. This is a major deal.