While entering the options menu, hold D to access hidden options that allow changes to the speed, recovery type from attacks, and more.
At the main menu, highlight the "Arcade Mode" option, then hold D and press A to select it. If you entered the code correctly, the character selection screen background will turn blue. The best times that G-Akiha has been defeated will also be posted on a screen.
Successfully complete Arcade mode without losing any fights to fight Aozaki Aoko after White Len. Win the match to unlock hidden dialogue during the ending credits and an extra image of the character that defeated her.