Test Run:
You must tag all tag spots, beat all Street Challenges, and get all the Graffiti Souls to unlock the Test Run in each specific area. In the Test Runs, you can unlock more characters.

Hidden Characters:
A.ku.mu.: Get a "Jet" ranking in Fortified Residential Zone.
Beat: Defeat him in a street race.
Boogie: She can be found at Kibogaoka Hill.
Clutch: Find Clutch through DJ Professor K's instructions.
Combo: Successfully beat his challenges.
Cube: Go to the Bottom Point of Sewage Facility and out race Cube in the City Rush. This must be done after beating the Noise Tanks in Sky-Dinosaurian Square.
Doom Riders: Get a "Jet" ranking in Dogenzaka Hill.
Goji Rokkaku: Get a "Jet" ranking in Rokkaku-Dai Heights.
Garam: Successfully beat his challenges.
Immortals: Get a "Jet" ranking in Skyscraper District and Pharaoh Park.|
Jazz: Defeat Jazz at City Rush in the Future Site of Rokkaku Expo Stadium.
Love Shockers: Get a "Jet" ranking in Hikage Street.
Noise Tanks: Get a "Jet" ranking in Highway Zero.
NT-3000: Get a "Jet" ranking in Sky-Dinosaurium Square.
Poison Jam: Get a "Jet" ranking in Test Run mode at the Tokyo Underground Sewage Facility and the Bottom Point of the Sewage Facility.
Potts: Get a "Jet" ranking in Kibogaoka Hill.
Rapid 99: Get a "Jet" ranking in 99th Street.
Rhyth: Go to the top of the Poison Jam warehouse in Rokkaku-dai Heights to find her the first time. Follow and find her two more times in the game.
RoBoy: Get a "Jet" ranking in Chou Street. Note: A.ku.mu. must first be unlocked.
Soda: Beat Soda at City Rush in Highway Zero.
Zero Beat: Get a "Jet" ranking in Test Run mode at Shibuya Terminal.

Thanks to Revolution readers Marcus, Adam Dutton and The Notorious B.G.!