Fight as Sara, Bun Bun, Jacky, and Jeffry:
Complete the game with each character with normal or greater difficulty.

Fight as the Boss:
Set the difficulty level to hard, number of rounds to two or more, then complete the game in single player mode.

Big head fighters:
Enable all characters, including the Boss and four bonus fighters. Use each of these characters to complete the game to access a big head mode option.