Ok. Here's the scoop to all your questions even if they were already answered in the thread.
1. Shadow Mario may or may not be an official character now. He IS in MarioGolf: TOadstool Tour, and I own the game and unlocked him. To get him, you must complete all the ring courses in One-Player mode.
2. Shadow Mario is not his own character, and Bowser did not create him. Bowser Jr. (The latest addition to the Koopa family) simply purchased a special Paint Brush made by E. Gadd Enterprises (Or whatever his company's name is) and got a special Bandana from somewhere that can transform him inot Shadow Mario when he puts it over his face.
3. There are three classes of children of Bowser: Bowser Jr. who seems to be the most probable of the all the children and was introduced in Mario Sunshine, Koopa Kids/Baby Bowsers who are numerous and appeared in Mario Party games, and the Koopalings which were in Super Mario Bros. 3, Yoshi's Safari, Super Mario World, and Mario is Missing.
4.Toadette is not a new character. It is simply a female version of a Toad. Like Toad, it can be the actual character Toad, or simply the race of people. Toadtettes have appeared in numerous games which include Paper Mario and Super Mario RPG.
There. Now everyone stop arguing because you've been blessed by the wisdom of Meowrik this time!
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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!