The Wii Friend Code is Mii specific for all competitive online games - just like Mario Strikers Charged.
This means that multiple people can play the same game online on the same Wii using seperate accounts, and that you won´t have to add friends again for future online games if you´ve already added them for Mario Strikers.
Justin Dowdeswell, the producer of Mario Strikers Charged, confirmed this.
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