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: Unlock A.ku.mu., then 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.

Test Run option:
Successfully complete the game then talk to RoBoy to unlock the "Test Run" option. Earn "Jet" ranks to unlock the characters in test run mode.

First, find the cassette (gold with a red "?" on it) for the level you want to unlock test run on. Next, press Start and read GG-notebook. Clear all the graffiti and street challenges. For each one you clear, a new graffiti soul will appear and it will show you where it is located. Do all of those, then get the default graffiti souls (the ones that are there when you start the level). Note: Special events may happen in certain levels that you must complete as well as everything else.

Hint: Easy Jet Tech:
In each level there is a 500 point grind. For example, in any level with the green metal side walk guard gives you 500 points. If you do the back to front or Y grind combo over that same grind, you can easily get a Jet ranking.