Highlight the "Arcade Mode" option at the menu, then press A while holding D to select it. The character selection screen background will turn blue to confirm correct code entry. The best times that G-Akiha has been defeated will also be posted on a screen.
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.