Beth doesn't have much of a presence herself in the story, but she is a (wait for it… twenty dollar word coming up…) catalyst for the actions/ feelings of other characters. Okay, more like just one other character: Jed. Beth is Jed's wife and Travis and Harry's sister. She gets pregnant twice, and loses the baby both times thanks to zombies. She miscarries the first baby after dealing with the crazy stress of Jed's mom turning into a zombie, and her second baby dies along with her when she becomes a flesh-eating monster herself. Poor girl.
Her death, surprisingly, does little to shake Harry and Travis, and they continue on their merry zombie-slaying way. But it practically crushes Jed, who's completely in love with her, in addition to forcing him to focus again on his little sister, since she's the only family he's got left. What does that mean for Jed? He'll toss himself into a frenzy of zombies, and end up saving Mary but dying himself. What does that mean for Beth? In the end, her death saved Mary, but killed her husband.
That's rough, man.