Hmm.... it'd appear that he may be a new character. In a sense, Bowser Jr. is the most logical and useful of all the children Bowser ever had, because anytime Bowser needs a partner for something, Ie: Mario Kart Double Dash!!, Mario Golf: Toadstool Tour, or possibly Mario Tennis, it's easier this way. If they were to keep the Koopalings, then they'd have a hard time choosing which ones to use in certain games, which would spark up arguments and distaste for the game from fans of certain Koopaling Fans. And they wouldn't be able to waste space and put all the Koopalings in one game. So, I for one think Bowser Jr. should stay the new child of Bowser.
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I am Meowrik, the 5th Samurai Pizza Cat. My trusty Jalapeno Katana along with my Chili Claws will prove my might!