Lieutenant Kotler is a part of the Nazi system in The Boy in the Striped Pajamas.
Kotler's personality is not much else other than the Nazi system. He is described as having "very blonde hair, an almost unnatural shade of yellow." This is representative of the Third Reich's insistence on "blonde hair and blue eyes" embodying the "perfect" human being. Kotler's personality can also be very charming. This is seen in how he cultivates Gretel's affection and in the effect he has on Bruno's mother. At the same time, Kotler can be incredibly cruel. This is evident in the savage treatment Shmuel receives. Bruno watches in silence as Kotler abuses the boy. It is also seen in his treatment of other prisoners in "Out- With." Kotler's personality uses power over others as a way to enhance his own image.
Kotler is the closest thing to an antagonist in the novel. He is the representation of Third Reich. Accordingly, there is not much emotional depth to him. However, through Kotler, Boyne shows how evil can be so easily embraced. Gretel fails to acknowledge or wishes to deny any connection between Kotler and his morally reprehensible actions. In much the same way, when Bruno notes how his mother laughs more at Kotler's jokes than "Father's" or when she refers to him as "Kurt precious," it is evident that she fails to comprehend the horrible acts he initiates in the camp. The effect of Kotler's personality on other characters shows how individuals must be vigilant about evil.