Hidden characters:

A.ku.mu.:      Earn a "Jet" ranking at Fortified Residential Zone.
Beat:          Defeat him in a street race.
Boogie:        She can be found at Kibogaoka Hill.
Clutch:        Follow DJ Professor K's instructions.
Combo:         Successfully complete his challenges.
Cube:          Defeat the Noise Tanks at Sky-Dinosaurian Square, then go to the
               bottom of the Sewage Facility and defeat Cube in the City Rush.
Doom Riders:   Earn a "Jet" ranking at Dogenzaka Hill.
Garam:         Successfully complete his challenges.
Goji Rokkaku:  Earn a "Jet" ranking at Rokkaku-Dai Heights.
Immortals:     Earn a "Jet" ranking in Skyscraper District and Pharaoh Park.
Jazz:          Go to the future site of Rokkaku Expo Stadium. Defeat Jazz at
               City Rush
Love Shockers: Earn a "Jet" ranking at Hikage Street.
Noise Tanks:   Earn a "Jet" ranking at Highway Zero.
NT-3000:       Earn a "Jet" ranking at Sky-Dinosaurium Square.
Poison Jam:    Earn a "Jet" rank in a test run at the Tokyo Underground Sewage
               Facility and at the bottom of Sewage Facility.
Potts:         Earn a "Jet" rank at Kibogaoka Hill.
Rapid 99:      Earn a "Jet" rank at 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:         Earn a "Jet" rank at Chou Street.
Soda:          Defeat Soda in City Rush at Highway Zero.
Zero Beat:     Earn a "Jet" rank in a test run at Shibuya Terminal.