Bruno is the narrator of The Boy in the Striped Pajamas and he has a very definite opinion of his older sister, Gretel. He refers to her as "the hopeless case," right from the beginning (chapter 1) suggesting that "no one needed Gretel much." She belittles him and pretends to be older than she is, until Bruno puts her in her place around Lieutenant Kotler.
In chapter 14 it is raining and Gretel comes to Bruno's room because she is bored. Bruno is unimpressed with the interruption; however, he shows that he is maturing and decides to try and be more courteous than he would normally be because "even brothers and sisters can lay down their instruments of torture..." He understands that the rain is a nuisance and, before he can stop himself, he mentions how the rain prevents him from meeting with Shmuel which piques Gretel's interest.
Unfortunately, there is no sense of trust between the siblings, and so when Gretel will not stop asking Bruno about what he has said, he has to think quickly and tells Gretel that he has an "imaginary" friend. Their relationship is such that Bruno has to accept the teasing that will surely follow because that is the only way to preserve his secret. Gretel now calls Bruno "a hopeless case." There is a brief opportunity for the siblings to support each other and Bruno even thinks that she is coming around when she mentions her own imaginary friend. She is, however, only teasing him, completely overlooking the fact that she plays with dolls, and so in fact has her own set of imaginary friends. Gretel does not see it that way.