Unlock Every Character:
Go to Character Customization and select "Name Selection." Quickly enter L1, R2, R1, L2, L1, R2, R1, L2, Square, then press Right or Left to toggle between new characters.

Hard mode:
Get 1,000 points.

Sudden Death mode:
Get 7,500 points.

Thanks to Revolution readers KAMIKUZEXXX, Eddie Alford, Sarah Thustra, Freakytehfrog, and Jeremiah Oglesby!