How do you unlock all of the characters?

There are a ton of characters I haven't unlocked yet. Are they just unlocked by completing Grand Pris?

9 of the unlockables are unlocked by playing the cups from the normal game, and the other 6 are unlocked by playing custom cups

Croassung is correct, I highly recommend this multicup for unlocking the custom cup characters (make sure to use Grand Prix mode)
https://mkpc.malahieude.net/circuit.php?mid=6816

fun fact: i still haven't unlocked them all bc it's pointless

thanks!!

It also depends on how many you had won in order to unlock them in grand prix and custom mode