J. J. Sawyer:
Successfully complete the game with an "S" rank in all levels.

Naomi:
Successfully complete the Downtown level with a "B" rank.

Oscar:
Successfully complete the game with an "A" rank in all levels.

The Insider:
Successfully complete the Storage level with a "B" rank.

Stunt attack mode:
Successfully complete the game in story mode.

Alternate costumes:
Complete one of the following tasks to unlock a third costume for the indicated character.

Ethan: Successfully complete stunt attack mode.
Kat: Successfully complete score attack mode with high scores in all levels.

Glitch: Fall through door:
Unlock Naomi, select freestyle mode and go to level three (Industrial). At the end of the level, go off the left ramp from the exit (at the search lights). Avoid the big pipes, use Speed and Boost Jump, and you should fall through the door and float in darkness.