Unlock the real-life riders by beating these challenges:
#10 ROBERTS JR, Kenny (Usa) Challenge #5
#15 GIBERNAU, Sete (Spa) Challenge #14
#17 VAN DER GOORBERGH, Jurgen (Ned) Challenge #21
#19 JACQUE, Olivier (Fra) Challenge #90
#20 RIBA, Pere (Spa) Challenge #17
#21 HOPKINS, John (Aus) Challenge #18
#3 BIAGGI, Max (Ita) Challenge #10
#31 HARADA, Tetsuya (Jap) Challenge #3
#4 BARROS, Alex (Bra) Challenge #9
#46 ROSSI, Valentino (Ita) Challenge #12
#55 LACONI, Regis (Fra) Challenge #19
#56 NAKANO, Shinya (Jap) Challenge #11
#6 ABE, Norick (Jap) Challenge #4
#65 CAPIROSSI, Loris (Ita) Challenge #16
#7 CHECA, Carlos (Spa) Challenge #7
#74 KATO, Daijiro (Jap) Challenge #6
#8 McCOY, Garry (Aus) Challenge #13
#9 AOKI, Nobuatsu (Jap) Challenge #91
#99 McWILLIAMS, Jeremy (UK) Challenge #20

Beat those challenges to unlock those fictional riders:
Hitomi Yoshino - Challenge #85
Jack Slate - Challenge #62
Susumu Hori - Challenge #1

Unlock the Legend riders by beating Challenges:
DOOHAN, Mick (Aus) - Challenge #15
GARDNER, Wayne (Aus) - Challenge #87
RAINEY, Wayne (Usa) - Challenge #2
SCHWANTZ, Kevin (Usa) - Challenge #12