Play through the game (beat ''Final Stage'') and save. Professor K can now be chosen as a playable character. He is located between Garam and Beat on the character selection screen although his image is static.
Unlock Poison Jam
Beat all Kogane-Cho levels with a Jet ranking.
Change Sega Intro
Start the GBA with Jet Grind Radio and continuously press the A button until the Sega logo appears. You will hear a person scream the word Sega instead of the normal longer chorus.