This is an interesting question focused on only one of the narrators of this disturbing novel (about a bus to summer camp hijacked by terrorists). Kate, of course, is the sixteen year old substitute bus drive on that fateful day. As such, Kate is a narrator that is subject to both direct and indirect characterization. Let's define the terms first, and then see how they apply to the character of Kate.
An author can use either indirect characterization or direct characterization; however, an author can also use both indirect characterization and direct characterization. Cormier decides to use both. Direct characterization is when a character trait is specifically used to describe a character. For example, when other narrators say that "Kate is brave," that would be an example of direct characterization. Indirect characterization is when the character's traits are revealed by what he or she says or does. In this case, Kate trying to save the children (and any of her actions in that regard) or her thoughts that imply bravery would be considered indirect characterization.
Because Kate is a narrator among narrators, the author uses indirect and direct characterization to reveal her personality. All of the times the other characters describe her as "brave," and even when when Miro says that Kate "betrays" him, those are both instances of direct characterization. There are also examples of indirect characterization in regards to Kate. In this case, Kate tries to drive the bus backwards in order to escape:
If she went off course, the parapet looked strong enough to prevent the bus from crashing off the bride [into the] river below.
This is an action that Kate performs that proves her as brave in regards to the five year old children she is trying to save. Even though the words don't say that Kate is brave, her actions speak the part.
Thus, both types of characterization are used by Cormier in the character of Kate. Even a famous quotation from the book could be considered indirect characterization about Kate:
The possibility that hope comes out of hopelessness and that the opposite of things carry the seeds of birth - love out of hate, good out of evil. Didn't flowers grow out of dirt?