Hi

I'm the Art director of STK so you reached the right person

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Let me explain Currently there is no "strong rules" about characters. There is basically "2 levels" in which a character can be released.
* You can do your own character and simply submit it to addons.supertuxkart.net. It will be reviewed (we can't accept offensive, violent content, etc) and if it passes these simples tests well everyone with the game and an internet access can download and play with your characters (you can even do tracks with the same system).
* If your character is good enough and if you ask us we can eventually include him/her in the core game. Which means he/she will become officially part of SuperTuxKart.
Now to be included in the addons it's basically we include everything as long as it's not offensive/violent/etc. To be included in the core game, the main rule is it should be an open source project mascot and the quality should be at least equivalent to Tux, Amanda, Gaveroche, Sara (racer and wizard), Suzanne (in 0.9). They are our references. Otherwise it's mainly, if you want a character do it yourself.
As I understand, you want us to make your mascot as a playable character in stk ? Now you might not realize it but making a new character is kinda hard and takes some time, especially if it's your first one. If we do your character it will be because one of our artist is interested or maybe (but that never happen) be paid to do it.
So your character is welcome, but it's not sure a SuperTuxKart artist will do it. It takes time and dedication to do a new character, so not sure what will happen

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I hope I answered to your questions.
Btw we included not well known character in stk like Amanda, the Panda. Tux, gnu are probably the most popular, but they aren't the only ones.
Sam