I have Sonic Riders. Challenging game. Here is every character, saying what they use, and how to unlock them.
Sonic - Default - Boards, Skates, Bikes
Tails - Default - Boards, Skates, Bikes
Knuckles - Default - Boards, Skates, Bikes
Amy - Default - Boards, Skates, Bikes
Jet - Default - Boards, Skates, Bikes
Wave - Default - Boards, Skates, Bikes
Storm - Default - Boards, Skates, Bikes
SPOILERS
from now on
Eggman - Beat Babylon Garden - Bikes Only
Cream - Beat Hero Story - Boards, Skates, Bikes
Rouge - Beat Hero Story - Boards, Skates, Bikes
Shadow - Beat Hero Story - Boards, Skates, Bikes
NiGHTS - Beat Every Mission - Boards, Skates (?), Bikes (?)
Ulala - Beat Every Mission - Boards
AiAi - Beat Every Mission - Boards, Skates (?), Bikes (?)
E-10000G - Play for 20 Hours - Boards
E-10000R - Play for 50 Hours - Boards
Super Sonic - Get Gold in every mission - N/A (Boards?)
I can't remember NiGHTS pr AiAi's skills
END SPOILERS
Truthully, the characters aren't very unique. You can play as any and get the same record. Now Boards on the other hand...