Bruno's character grows throughout the novel The Boy in the Striped Pajamas as he learns to distinguish people who are good from those who are not. For instance, when his sister is forming an attachment to Lieutenant Kotler, Bruno is unhappy. He recognizes that Lieutenant Kotler is not a nice person.
Bruno feels “a great urge to go back there and pull Gretel away” from Lieutenant Kotler. It is not that Bruno and Gretel have gotten closer than they were in Berlin. In fact, he thinks that "she [is] annoying and self-centered and mean to him most of the time." He recognizes, however, that this is normal for siblings at a certain age. He even thinks to himself, “That, after all, was her job. She was his sister.”
The reason that he does not want Gretel to get close to Lieutenant Kotler is that,
he hated the idea of leaving her alone with a man like Lieutenant Kotler. There really was no other way to dress it up: he was just plain nasty.
Another way that Bruno grows is that he learns to be more adaptable. When the book opens, Bruno is unhappy about leaving Berlin and his friends. He does not understand the need for the move and he protests to his mother. By the end of the book his parents decide that he and his sister will return to Berlin. Bruno does not really want to leave because of his friendship with Shmuel, but he decides not to protest. He “decided that whatever happened, he would accept the decision without complaint.”
He also comes to realize that home is relative. To him, it is where you have good friends. He tells Shmuel:
. . .The rest of us are going home.' He said the word 'home', despite the fact that he wasn't sure where 'home' was any more.
When his father asks how he feels about the move back to Berlin, he even replies:
Well, yes,' he replied, considering his answer carefully, "but I think I'd miss people no matter where I went.
Despite his youth, he is astute enough to realize that he can form important new friendships anywhere, as he has with Shmuel. He also realizes that things probably have changed in Berlin:
There was one part of him that remembered that he had loved his own life back there, but so many things would have changed by now. Karl and the other two best friends whose names he couldn't remember would probably have forgotten about him by now.
The day before the planned departure, Bruno also agrees to help Shmuel find his father because he had promised that he would, and "he wasn't the sort to go back on a promise."