Hidden Character Code
The characters Rubber Soul, Khan, Shadow Dio, New Kakyoin and Jojo are hidden by default, however, you can access them by using a code.
1) Access the Diagnostic menu (F2 if you're using FinalBurn Alpha)
2) Select Configuration
3) Select Game
4) Press Up, Down, Right, Left, A, B, C, S
Note: make sure you do NOT have MK/HK, or 2P controls overlapping with your P1 buttons or those will interfere) A sound will confirm that you have entered the code correctly.
Hidden Characters on Select Screen
There are two hidden squares (different from the hidden characters) that seemingly used to contain the boss characters. These now result in crashes, but you can still try to select them if you want!
1) Highlight Abdul/Kakyoin
2) Hold Left (Abdul) or Right (Kakyoin)
3) Press Up
Speculation is that this may also be the character "Grey Fly", who appears in debugging mode as a scrambled mess of Jotaro and Vanilla Ice.
This isn't really a secret, because you'll probably only be able to do this on an emulator. Region 0x00 is the hidden debug mode, where you can mess with things like boss characters and hidden stages.
In FinalBurn Alpha:
1) Press F5 (controls menu)
2) Double click Region
3) Enter 0x00
4) Press F3 (Reset)
When in-game, you now have access to the following:
Start+LP/MP -- Switches stages
Start+HP -- Time stop color switch
HP+Start -- Character switch
P1 MK -- Pause
P1 HK -- Slow-Mo/Advance Frame
Boss Dio can only be used in one player gameplay. In 2P mode, he reverts to regular Dio.
Development Mode aka. True Debug Mode
This will not work on the ggpo version of Final Burn Alpha use a newer version.
Warning you can edit some the game's quality. So in intrest of Stand Crash we do not recommend you to do this
There is another mode that the developers used for testing the backgrounds looking at sprites, frames of animation, and other misc. things. To access development mode use 0x70 in region settings (on input mappings screen for fba). The second part is to reset and hold A then enter hit the service menu button(F2).
Hidden through out the arcade roms is a lot of unused artwork. All this section would just tell our speculation about these unused assets.