Unlock Naomi:

Get B rank on level 2

Unlock Insider:

Get B rank on level 4

Unlock J.J. Sawyer:

Beat the game with an S rank in all levels.

Unlock Oscar:

Beat the game with an A rank in all levels.

Stunt attack mode:

Beat Story Mode.

Unlock Ethan costume C:

Beat Stunt Attack.

Unlock Kat costume C:

Beat Score attack:

Unlimited Boost:

Beat the game in Story mode, Score attack, and Stunt attack.

Always on Top of the Score List:

Choose New game and enter your name as Ethan and the PS2 will think you've completed the game.