Seppi Deppi Duck (German name) is Donald Duck's brother.
Little is known about Seppi Deppi Duck, other than the fact that he is Donald and Della's brother and thus probably Quackmore and Hortense Duck's son. He looks a lot like Donald, although he seems noticeably fatter and, as far as one can tell from his portrait, less well-mannered. His brother owns a picture of him, which hangs in his house.
Behind the ScenesEditThe character of Seppi Deppi was actually at first created as a translation change. In the German rescripting of Barks's story High-wire Daredvils by Erika Fuchs, Donald briefly mentions "Huey Dewey and Louie's uncle Seppi Deppi", who hurt himself while tightrope-walking, in an effort to dissuade his nephews from becoming tightrope walkers themselves. If it had stopped to that, Seppi Deppi would have just been a curiosity, a mere translation mistake, definitely out of any canon you may find.
However, D.O.N.A.L.D.I.S.T. Johannes A. Grote made a Duck family tree based on what he believed was Barks's stories, but was actually Fuchs's rescripting. Seppi Deppi Duck was thus included and “officialized”. Grote's novel idea for his tree was to consider that all Duck portraits seen hanging in Donald's home were probably members of his family, and thus to match them with characters from whom he didn't have any official picture. Seppi Deppi was thus retconned as the tongue-sticking duck seen in a portrait in a yet unidentified Barks background.