Colonel Wilhelm Wilner was an old college friend of John Smith from before World War II, although he knew Smith as Johann Schmidt. He was also an old friend of Paul Kramer. He was known affectionately to both as "Willi-Willi." During the war, he served as Field Marshal Albert Kesselring's chief of staff in Italy. Smith, as Schmidt, served as a double agent for MI6. Using his prewar friendship with Wilner, he was able to convince the Nazis he was a loyal German spy, although he only ever fed them out of date information. Wilner was none the wiser. During the mission to learn the names of Britain's German spies from Olaf Christiansen, Lee Thomas and Edward Carraciola, Smith name-dropped Wilner as proof that he was who he claimed to be. Kramer double-checked, phoning Wilner, who confirmed to his old friend that he did indeed know Schmidt.
In the film, Wilner is a major and not a colonel.