I brought a second-hand 3DS and when in the Mii Plaza, I am the only character, the favorite. But when I play Mario Kart 7, all the accolades show as the previous owner's Mii even though her Mii is not to be found. How do I get the previous owner's Mii out of the system, and set all games to use my Mii?
Open up Mii Maker, select the button that says View Mii Characters, then tap on the Mii you want to get rid of and choose Erase. If it is possible to get rid of the Mii you don't want, this should do it.