It's the Mii! He takes the face of a random Mii that you own, for each fight.
Special moves:
B: Tennis: Mii throws a Tennis ball into the air, and then hits it. The direction the ball goes in can be slightly controlled. Deals moderate damage, and has little knockback.
B side: Baseball
If you tap the control stick in a direction, the Mii will throw a baseball, that deals a little damage. This can be used as a spam-attack. If you hold the control stick in a direction, Mii will swing a Baseball bat (kind of like Kirby's hammer). This attack reflects projectiles. This is the closest thing to a recovery that Mii has, and if used in the air, you will get a little higher, and a little farther.
B up: Bowling
Mii pauses for a two seconds, which this gives you time to aim this attack. Then, Mii throws a bowling ball in the direction you aimed in. When the bowling ball lands, it will roll in the direction aimed, and if anyone is hit by the ball while it is rolling, they will suffer a lot of damage, as well as a lot of knockback.
B down: Golf
Mii takes a random golf club (meaning it changes, like Peaches side smash), and hits you with it. The type of club you got hit with, determines the damage you take, as well as the knockback. Beware of the Driver, as it is the most damaging.
Final Smash: Boxing
Mii gets boxing gloves, and he/she is indestructible for a short period of time (like Wario Man). With boxing gloves, Mii's Smash attacks doe extra damage, and they knock back the target farther. Mii also has his Smash attacks quickened, to the point where they have the speed of normal attacks, and they are a lot harder to dodge.
