Unlock all characters in Mario Kart Wii - Guide for Mario Kart Wii

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LIGHT WEIGHT
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Baby Luigi -
Unlock eight expert staff ghosts on time trial mode.
Win 100 Wi-fi connection ghost races.
Play games: 3,150.

Baby Daisy -
Score one star or higher in all Wii cups on 50cc grand prix.
Play games: 1,950.

Toadette -
Complete all courses on time trial mode.
Win 1,000 Wi-fi connection races.
Play games: 2,550.

Dry Bones-
Finish leaf cup in first place on 100cc grand prix.
Play games: 1,050.

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MIDDLE WEIGHT
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Daisy -
Finish special cup in first place on 150cc grand prix.
Play games: 2,850.

Birdo -
Complete 16 courses on time trial.
Win 250 Wi-fi connection races.
Play games: 1,350.

Diddy Kong -
Finish lightning cup in first place on 50cc grand prix.
Play games: 450.

Bowser Jr. -
Score 1 star or higher in all retro cups in 100cc grand prix.
Play games: 3,450.

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HEAVY WEIGHT
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King Boo -
Finish star cup in first place on 50cc grand prix.
Play games: 750.

Rosalina -
Score 1 star or higher in all cups on mirror grand prix.
Have a Super Mario Galaxy save data and play 50 games.
Play games: 4,950.

Funky Kong -
Unlock 4 expert staff ghosts on time trial.
Win 25 Wi-fi connection ghost races.
Play games: 2,250.

Dry Bowser -
Score 1 star or higher in all Wii cups on 150cc grand prix.
Play games: 4,350.

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Mii characters
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Mii racing suit -
Finish special cup in first place on 100cc grand prix.
Play games: 1,650.

Mii plumber's costume -
Unlock all 32 expert staff ghosts on time trial.
Win 5,000 Wi-fi connection ghost races.
Play games: 5,100.
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Just some info on the Mii characters. The weight class well be dependent on the 
type of Mii you use. If you use a Mii character with generally short height and 
small weight, you will get a lightweight Mii. If you use generally middle 
height and middle weight style Miis, it  will be middleweight. And for 
heavyweight, with generally high height and high weight, you will get your 
heavyweight mii. However, this does not effect the vehicle color. It will be 
white and blue I believe, and for the racing suit's color, it changes varying 
on your favorite color set for your mii.