A small list of the modes you can unlock each Character in.
Characters
== Starting Characters ==
SpongeBob
Patrick
Sandy
Leo/Leonardo
Raph/Raphael
Donnie/Donatello
Mikey/Michelangelo
Lincoln
Lucy
Tommy
Chuckie
Arnold
Helga
Rocko
Zim
Aang
== Unlock in Grand Prix ==
Unlock 1 character for getting 1st in each race of Grand Prix.
CatDog
Gerald
Angelica
Squidward
JoJo Siwa
Ren
Clyde
Heffer
== Unlock in Challenges ==
Defeat each Character in Challenge Mode to unlock them
Danny Phantom
Stimpy
Korra
GIR
Reptar
Shredder
By hurt138