The character description provided does not specify any further information about the 25-year-old German man. In terms of Germans living in Poland, the situation is complicated because Germany invaded Poland in 1939 and then occupied the territory. Many of the Germans who were sent to live in Poland were members of the military. Hitler’s orders in seeking “living room” for Germans included killing people of Polish heritage and also endeavored to split up the ethnic groups so they would not unite against the invaders. In contrast, many people of German descent had already been living in Poland for generations; some of them were Jews. The character’s attitudes and behaviors will be influenced by the length of his and his family’s residence as well as their religion.
Hitler supported a policy of “depolonizing” and “Germanizing” Poland quickly. A character who was active in the German military would likely have responsibilities for arresting and imprisoning Polish people, including large numbers of Jews, and transporting them to forced labor or concentration camps such as Auschwitz. There, many soldiers would have been guards.
People were also moved around the conquered territory, with Poles removed and ethnic Germans imported from areas around the Baltic. Along with Jews, Catholics were targeted, and German soldiers destroyed churches and killed priests. Resistance was ongoing and culminated in the Warsaw ghetto uprising of 1943, which was met with severe reprisals.
In terms of sources, the historical documentation includes descriptions of atrocities that soldiers committed, even massacring entire villages. Recently uncovered recordings reveal conversations among German soldiers, sometimes made by the Allies in the cells of prisoners of war.