While playing through the cups I recommend switching characters each time you start a new cup. That way you can also try to knock out all the character specific achievements.
To unlock this achievement you will have to win a race with all 12 characters, so this includes the locked ones and the 3 which are existing characters with an alternate costume/stats.
You will already have these, so win a race with each one:
Shrek
Alex
B.O.B
Skipper
Donkey
Hiccup
Unlock the remaining ones as follows:
Toothless - complete wind cup 50cc
Marty - complete moon cup 50cc
Fiona - complete one challenge
Alternate Shrek - complete all circuits in 100cc
Alternate Skipper - complete all circuits in 150cc
Alternate Alex - complete all mirror circuits.