Doon changes more than any other character in The City of Ember. At the beginning of the story, he is committed to saving Ember; he is quiet, moody, and more inclined toward mechanics than people. By the end, he is more confident and willing to look toward a life of uncertainty.

When Doon gets the job of Messenger after graduation, he gladly switches with Lina. His dream is to fix the generator and ensure that Ember continues to get power. He quickly realizes that this isn't the way to save the city. Later, when he's still trying to find ways to solve problems for Ember, he's classified as a threat to the city.

This classification is the one thing that really changes Doon. It forces him to leave the city along with Lina and Poppy. If their news and findings had been met with positivity and acceptance, he likely would have stayed in Ember. Instead, he's forced to leave and seek a life outside the city that sheltered them. His curiosity and desire to make the community better are punished, and this must disillusion him somewhat as well.

In the end, though, Doon still tries to give back to Ember. Though he, Lina, and Poppy can't go back, he tries to find a way to communicate what he's learned to the rest of the people there. Though he's changed in that he was willing to leave and open up more to Lina and Poppy, he is still a caring person and wants to make sure that others in Ember aren't left in the darkness.

The two main characters, Lina and Doon, would have to be considered the ones who grow and change the most throughout "The City of Ember".

Growing up in the underground city, it was their whole world. Life was as life had always been, as far as they were aware. Children grew up, took their assigned jobs, and lived out their lives just as their parents and past generations had done.

When they discover the note that reveals to them, after much work, that there is a way to "escape the city", another step of growth is taken as they contemplate that their known world is not necessarily all there is. This lesson goes even further when they run into the mayor and his helpers who try to stop them from spreading their "harmful ideas."

Finally, when they emerge from underground, they discover that they have been living below the surface of a much larger and glorious world. Sending the note back down wrapped in her baby sister's shirt is their attempt to help the ones left behind to share in their growth.