Detective Casey is Chief O'Hara's head detective, first appearing in the Mickey Mouse daily comic in the 1938 sequence, "The Plumber's Helper". The story was plotted and pencilled by Floyd Gottfredson and written by Merrill De Maris. Casey disappeared from American comics in the 1950s, but was used frequently in Europe, especially in Italy, afterwards; from 2003, he returned as a frequent player in the American comics once again.
Detective Casey first appeared in "The Plumber's Helper" (1938) when Mickey lost all of his money in a stock market crash and is forced to go to work as an apprentice to a very shady plumber named Joe Piper. His next important appearance is in "Mickey Mouse Outwits the Phantom Blot" (May 20 - September 9, 1939) in the tale that introduced the Phantom Blot. While appearing in several more early MM stories, he has since become more prevalent in European Mickey Mouse tales.
Despite his occupation, Casey is an impatient man of only average intelligence. Thus, while sometimes a successful detective, he is prone to bungling cases as well. Therefore, Chief O'Hara often recruits Mickey Mouse to help solve some of Casey's cases, much to Casey's general irritation.