From the Wolfenstein: The New Order in-game wiki
Born in Strasbourg, France, Friedrich Keller's childhood was defined by domestic abuse and poverty. When Germany invaded France, Keller's father joined the resistance. Tired of living in poverty, Keller wnet to the local SS office and informed the Nazis of his father's ties to the resistance. His family was rounded up and sent to Auschwitz, but Keller was taken under the wing of the notorious General Wilhelm Strasse, whom he has served ever since. Keller views General Strasse as his surrogate father.