Please note that this guide was currently compiled by me going through the game myself. I do not know if most of these unlock just from Arcade mode or Story mode or both. I will tell what rank I got (in case it matters), but please feel free to let me know if you got any of these by different methods (such as just completing the stage) and I can amend the guide. Also, if you know which stage unlocks which Merenstein VI color, let me know and I'll add it in.


Planes:

Merenstein VI
-Complete the 1st stage on Arcade mode. I got a rank of A on challenging difficulty.
-Complete the 3rd stage on Arcade mode. I got a rank of A on challenging difficulty.
-Complete the 7th stage on Arcade mode. I got a rank of C on challenging difficulty.



BS4-VR Soprano
-Finish the alternative story mode on any difficulty to get the black/grey Soprano.
(Complete the story mode on challenging difficulty to unlock alternative story mode.)



GE-22 Liberator
-Complete the 5th stage on Arcade mode to get the grey & yellow Liberator. I got a rank of A on challenging difficulty.





Art:

Art Filter
-Complete the 4th stage on Arcade mode. I got a rank of A on challenging difficulty.


Phosphorus Art
-Complete the 6th stage on Arcade mode. I got a rank of C on challenging difficulty.



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Here is the guide in order of stages:

-Complete stage 1 on Arcade mode = Merenstein VI alternative color
-Complete stage 2 on Arcade mode = ~nothing unlocked~
-Complete stage 3 on Arcade mode = Merenstein VI alternative color
-Complete stage 4 on Arcade mode = Art Filter
-Complete stage 5 on Arcade mode = GE-22 Liberator alternative color (grey & yellow)
-Complete stage 6 on Arcade mode = Phosphorus Art
-Complete stage 7 on Arcade mode = Merenstein VI alternative color


-Complete the story mode on challenging difficulty = unlock alternative story mode
-Complete the alternative story mode on any difficulty = BS4-VR Soprano (black/grey version)