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John D. Rockerduck is an anthropomorphic duck.
John D. Rockerduck is the second richest duck in the world (in a tie with his rival Flintheart Glomgold), with both of them being just a little below Scrooge McDuck. Unlike Glomgold, John isn't simply an "evil twin" of Scrooge. Instead he has completely different values, as evidenced by the fact that, unlike his two rivals, he is an unreasonably big spender, using up much of his fortune to show off thanks to luxurious buildings, yachts and the likes; Rockerduck is always one to ridicule Scrooge for not "using" his money enough. However, he has in common with Glomgold the wish to top Scrooge's fortune, and the willingness to use ruthless or even illegal means to fulfill this goal. At one point he even assumed the villainous persona of Rollerdollar, a member of the Bad Seven.
Behind the scenes
Rockerduck, created by Carl Barks a one-shot character in his story Boat Buster was picked up by other writers and made into their version of Scrooge's archnemesis and business rival, and thus said to be the "second-richest duck in the world". These writers were unfortunately not aware of Barks's earlier creation, Flintheart Glomgold, also described as the "second-richest duck in the world". The Duke of Baloni is also stated in his debut to be second-richest, though it's generally understood that he only briefly held the title, as confirmed in the unlicensed comic story Hare-Brained Hooligans.
In an effort to distinguish him from Glomgold, it is sometimes held that Rockerduck was younger than Glomgold and Scrooge. His age was ambiguous in his original few appearances, and at least one story holds Scrooge and Rockerduck were born on the same day. However, it should be noted that in Rosa's Raider of the Copper Hill, Rockerduck is implied to be at least ten years younger than Scrooge, and by extension Glomgold.