Fight G-Akiha anytime
To fight a boss normally only fightable at the end of one story route (G-Akiha), highlight the Arcade Mode option on the menu, then hold [D] and press [A] to select it. You will know this will have worked because the character select background will be blue; and the game will have a 'record' screen for the best time you achieve in defeating G-Akiha in this mode.
Story Mode Endings
The following covers how to get every ending on your list for Story Mode. You can view the endings you have unlocked by going to Options and looking at the right side of the screen while the second 'set' is highlighted. (If you are having trouble clearing Story Mode, you can alter the first option on the Options screen - difficulty level - and raise the number of continues to 4. Some routes are harder than others to complete). The instructions only cover the minimum required wins/losses - you still have to complete the last stage(s) of each storyline in order for the ending to be recognised.
1 - Night ON THE BLOOD LIAR ('True' ending) You can't get this ending until you've obtained endings 2-6. Do not lose any stages at all
2 - AGITATOR Win 'enter' and 'Sion Eltnam Altasia', then lose either 'Alice's Anxiety' or 'Stardust'. Or, if you can't go to Ending 1, you'll go here if you try.
3 - GOODBYE Win 'enter' but lose 'Sion Eltnam Altasia', then win 'Alice's Insanity'
4 - Hologram Summer Lose 'enter' and 'Extra Alchemist', then win 'Battle Princess'
5 - Freaks Channel Win 'enter', but lose 'Sion Eltnam Altasia' and 'Alice's Insanity'
6 - Other Tale Lose 'enter', 'Extra Alchemist' and 'Battle Princess'
7 - G. Lose 'enter', but win 'Extra Alchemist'
8 - oh my sister! ('Secret' ending) After getting every other ending, complete the 'G.' storyline ending again - afterwards, you'll see a hidden scene and have to win one last fight.
Fight Aozaki Aoko
A hidden boss awaits you in Arcade Mode. To fight Aozaki Aoko, you will have to complete Arcade mode without losing a single fight; after the battle with White Len you will get the chance to fight her. If you win, you will get treated to some bonus dialog by characters during the credits, plus a bonus image of the character you beat her with. If you lose, however, you cannot continue; you will have to restart. Note that due to how powerful her attacks are, if you are playing on easy difficulties, Aoko's AI is very unlikely to actually attack you, even if you stand around doing nothing, making the battle rather comical.