how to unlock them

s= small karts m= medium karts l=large karts
baby mario s you start with this character
baby peach s you start with this character
koopa troopa s you start with this character
toad s you start with this character
mario m you start with this character
luigi m you start with this character
peach m you start with this character
yoshi m you start with this character
wario l you start with this character
waluigi l you start with this character
donkey kong l you start with this character
bowser l you start with this character
king boo l Win the 50cc Star Cup
Diddy Kong m Win the 50cc Lightning Cup
Baby Daisy s Earn at least a 1 star rank in all 50cc Wii Grand Prix cups
Dry Bones s Win the 100cc Leaf Cup
Bowser Jr. m Attain at least a 1 star rank in all 100cc Retro Grand Prix cups
Daisy m Win 150cc Special Cup
Dry Bowser l Attain at least a 1 star rank in all 150cc Wii Grand Prix cups
Rosalina l Involves super Mario galaxy save data but she will unlock almost randomly as some report on only booting up the game unlocks her when others get her when they complete grand prixes

This is very helpful!

thx for help!