Over the course of the book, Mattie develops from a regular teenage girl into a mature young woman.

When the story begins, Mattie has to be roused out of bed by her mom, who needs her to do some chores. But Mattie is incredibly lazy and deeply resents having to do anything that might resemble hard work. Contrast this with how she behaves later on in the story when her mom goes missing. Under these trying circumstances, Mattie is forced to take on the running of the coffeeshop, which she does with remarkable efficiency. There's certainly no sign of the lazy young adult we saw earlier.

Mattie also develops emotionally throughout the story. To some extent, this is a result of the empathy that she's gained from tending to the sick. At first, she gives the impression that she regards Nathaniel Benson as a good friend but nothing more. It's only later on, when she discovers than Nathaniel didn't die from the fever, that she realizes she's developed deep feelings for him.