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Hmmm.....I forget what you have to do to get the secret characters, but to activate them, you have to go into the cheats menu, turn on the option for Alternate Characters (or something like that), and then hit a button when selecting your character (I believe it is the SELECT button). Basically, the alternate characters are merely a different skin and voiceover for whatever character you choose (the fighting does not change). Here the list for the alternate skin for each character:

Grave - You get Grave's outfit from the first game.

Juji - You play as his dear brother, Bunji.

Rocketbilly - You play as Garino....I think. It's been awhile since I played OD.