Hold Down + LP + MK + HP when reaching Sean's bonus stage for a harder difficulty. Hold Up + LK + MP + HK when reaching Sean's bonus stage for level 5.
Extra costume colors
Hold start when you've highlighted the character, then press any of the three punches or kicks to get a hidden costume. You must have beaten the game with a character to use any of his/her hidden costumes. To use the secret costume and press LP, HP, and MK. (beating the game with the character is not required for this costume)
Extra options
Successfully complete arcade mode as Gill.
Fight as Gill
Successfully complete the game with every character with the system direction set to "Normal". Gill will appear in between Yun and Yang at the character selection screen.
Fight Q in Arcade Mode
To Fight Q in Arcade Mode don't lose a single round in Arcade mode up till the end of Stage 8. If you die, you cannot rematch him.
System Direction page 10
Successfully complete the game with fifteen characters with the System Direction set to "Normal".
System Direction page 8
Successfully complete the game with five characters with the System Direction set to "Normal".
System Direction page 9
Successfully complete the game with ten characters with the System Direction set to "Normal".