How do you unlock characters on sonic robo blast 2

You don't. In order to gain new characters you must download .wad files others have made. Then you save it to the same place you saved the game when you downloaded it. Next, open the game, and on the title screen press Esc to get out of the menu and then press the ~ key to bring down a field where commands can be entered. Type "addfile wadfile.wad" and WHAMMO! You have added a character wad. Just be warned, do not enter ANY wads until you have cleared it 100%, which means all secret levels, all chaos emeralds, and even playing the very hard difficulty and clearing that too (you get a higher difficulty for that, and clearing it gives you more levels to clear). Then, only then is it safe to add wads. Why? As soon as you add a wad, the game becomes "modified" and you are barred from obtaining any secrets or seeing your statistics. And that is all there is to it.