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You can use Mii numbers to share your Mii characters with others or add those created by other players.
Share Mii numbers:
1. Tap a Mii icon displayed on the Manage Mii Characters screen.
2. Tap Share to share the Mii number.
Note: The options available when sharing will differ depending on your device.
Add Mii characters with Mii numbers:
1. On the Add a Mii screen, input a Mii number that someone has shared with you into the Mario Kart Tour Mii number box.