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You have to complete certain cups, and do certain trials but you can only get them on single player!
Loads!
Get gold trophy in grand prixs to unlock karts and characters.
For the karts the Wild Wing. For the bikes the Mach Bike.
you have to complete certain things to unlock stuff. this website shows you how to unlock everything, including karts. http://www.videogamesblogger.com/2008/04/18/how-to-unlock-all-mario-kart-wii-secret-characters-bikes-karts-and-more-guide.htm
COOL ones!
To get Rosalina, you have to have a New Super Mario Galaxy save file. Also, you have to beat time trials to unlock certain characters and bikes.
keep playing
That's completely opinion based.
Karts go faster, but bikes can wheelie for more speed.
You do an amount of races or you can finish cc's.
quit then go back to your players card.then click on it.the it will say you have unlocked the blue falcon