Hidden Characters:

A.ku.mu: Get Jet ranking in Fortified Residential Zone.

Beat: Beat him in a Street Race.

Boogie: Locate her at Kibogaoka Hill.

Clutch: Follow the Professor's instructions to get Clutch. 

Combo: Beat Combo's challenges.

Cube: Beat the Noise Tanks at Sky-Dinosaurian Square,
then head to the Sewage Facility and beat Cube in the City Rush.

Doom Riders: Get a "Jet" ranking at Dogenzaka Hill.

Garam: Beat Garam's challenges.

Goji Rokkaku: Get a "Jet" ranking at Rokkaku-Dai Heights.

Immortals: Get a "Jet" ranking in Skyscraper District/Pharaoh Park.

Jazz: Visit the future site of Rokkaku Expo Stadium,
then defeat Jazz at City Rush.

Love Shockers: Get a "Jet" ranking at Hikage Street.

Noise Tanks: Get a "Jet" ranking at Highway Zero.

NT-3000: Get a "Jet" ranking at Sky-Dinosaurium Square.

Poison Jam: Get a "Jet" rank at the test run of the Tokyo
Underground Sewage Facility and at the bottom of Sewage Facility.

Potts: Get a "Jet" rank at Kibogaoka Hill.

Rapid 99: Get a "Jet" rank at 99th Street.

Rhyth: Go to the top of the Poison Jam warehouse in Rokkaku-dai
Heights to find her. 

RoBoy: Get a "Jet" rank at Chou Street.

Soda: Beat Soda in City Rush.

Zero Beat: Get a "Jet" rank at the test run at Shibuya Terminal.