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.

Thanks to Revolution readers teie jansen and Simon&James!