Walter Donitz was Werner Donitz's grand-uncle. He was the youngest of four brothers.
Walter and his brothers (Rudolf, Joachim, and Friedrich) fought in separate branches of the Nazi army in WWII. Walter fought in the Heer, and served under General Walter Wenck in the 12th Army trying to reach Berlin. Walter surrendered to the Americans and survived the war.
Werner felt lingering shame for the fact that his grand-relatives were Nazis, and admitted to Pomson that was part of the reason he didn't feel worthy to propose to her. Pomson assured him that she didn't care what his ancestors did. (the Disease)