Instructions
Play through the game until you have completed Story Mode. Once you finish the game, you will see a message stating that Password Mode has been unlocked.
Restart the game and enter Password Mode. This mode is accessible via the main menu.
Type "bre7784erhe" into the password field using the virtual keyboard. This password alters your game file so that you've beaten every character.
Enter Story Mode and choose any character other than the one you selected to beat the game the first time. Complete Story Mode with the new character. Every time you complete Story Mode with a new character, you will unlock a new one for the roster. Repeat this process until you have unlocked every character.