I had the other characters from the very beginning, but not Jessie. They didn't have another role. One was a girl, by the way, and the other was a boy. Until I landed on Jessie, I didn't have a narrator who would jump off the page and come to life. It was partly because it was the next challenge for me as a writer, to imagine the world from a girl's point of view, but mostly it was because I thought that outdoor adventure novels needed to catch up to real life.
Outdoor adventure is every bit as appealing to girls as it is to boys.
On our own hikes and river trips we see at least as many girls as boys. I modeled loosely on former students of mine, others came from people who had been on river trips with us. When I'm starting to create a character, it helps me a lot to be able to picture a particular individual, to hear them talking, to visualize what they'd do in a situation. If I can begin with at least a few personality characteristics of an actual person, it immediately makes the character more real for me. Then I have something to build on.