Take the following steps to unlock the secret characters out of the 40 total Dragon Ball Z: Infinite World characters playable characters (not including transformations) in the PlayStation 2-exclusive versus fighting game.
To unlock those secret characters, you have to successfully complete the Saga they are in. Then, go to the Warrior’s Room. Go to the shop, and purchase them. When you have completed the Dragon missions, you can buy the new option labeled “?????” for 50,000 Zeni.
This video by Eduardo shows the full roster:
Thanks again to Narutovshaku for sending in the tip!