To unlock him, you have to defeat 100 Miis in Cruel Brawl. Afterwards, you have to face Reggie, who's 10 times more overpowered than any other Mii in that mode (you have to get him to %999 damage to knock him out, and every one of his attacks is a 1-hit K.O. against you) If you fail, he deletes all your Smash data, your SD card data, and your body, which was definitely NOT ready. If you beat him, you unlock him. He plays exactly like the Mii Fighters, but...better.
...why does Pit wear white after Labor Day?