You don't, because the Mii isn't a playable character. The Mii, however, will be a playable character in the next Super Smash Bros. games, Super Smash Bros. for Wii U and Super Smash Bros. for Nintendo 3DS.
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What character?
The fastest playable character in Super Smash Bros. Brawlis Sonic the Hedgehog.
No, Pichu is not a playable character in Super Smash Bros Brawl
No, Jack Skellington is not a playable character in Super Smash Bros. Brawl.
Super Castlevania IV happened in 1991.
Shadow is not a character in Super Smash Bros Brawl, unfortunately.
Midna is not a playable character in Super Smash Bros Brawl
Dark super Sonic is not a character in Super Smash Bros Brawl.
Yes, he will be a playable character in Super Smash Bros. for Wii U and Super Smash Bros. for Nintendo 3DS (both of which are Super Smash Bros. 4).
there is no main character