JoJo's Venture Hints & Cheats

At the character select screen do the following to make Dio appear:

highlight Alessy and press start
highlight D'Bo and press start
highlight Chaca and press start
highlight Milder and press start three times

At the character select screen do the following to make Shadow Dio appear:

highlight Alessy and press start
highlight D'Bo and press start
highlight Chaca and press start
highlight Milder and press start three times
highlight Dio and press start ten times

Enter the following at the character select for Young Joseph:

highlight Jotaro and press start
highlight Polnareff and press start
highlight Joseph and press start
highlight Kakyoin and press start
highlight Iggi and press start
highlight Avdol and press start
highlight joseph and press and hold start until Young Joseph appears

To unlock the time release characters all at once, you need access to the machine's test menu. From there, go to Configuration, then Game, and enter the following code. A sound will confirm it. Once entered, the characters will be permanently unlocked.

You need to get to the service menu to do this code. Go to Configuration, then Game, and enter the following code. You'll hear a sound to confirm it worked.