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Uncle Fester (or simply "Fester" when he introduces himself) is an unspecified relative of the Addams Family.
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Like all the Addams, he is attracted to the macabre.
The New Scooby-Doo Movies
After Gomez and Morticia left, Fester claimed he was the temporary head of the Addams' family. (This implied that he was not the head when they were both present, but would be the next in line in terms of authority.)