How do you unlock characters on sonic robo blast 2?

Answer 1

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.

Answer 2

SPOILERS INBOUND

There are three unlockable characters: Amy Rose, Fang The Sniper and Metal Sonic. I'll show methods of unlocking depending on the difficulty: easiest first, hardest last.

1: Fang

Fang is the easiest to unlock, AND is OP to most of the bosses.

Play the game up until Arid Canyon Zone 3. In there, Fang will appear as a boss for the zone. Beat him and whack the capsule to unlock him as a playable character.

2: Metal Sonic

Simple and straight forward: Beat the game, whether you have all Emeralds or not. Metal Sonic himself will make a cameo two acts before beating the game, in Black Core 1 and 2 as a boss.

3: Amy

Amy is the hardest to unlock, but can give a shield to others as a bonus in Multiplayer.

Get 5 emblems, be it Record Attack or all the collectable emblems in Greenflower Zone. You'll unlock Frozen Hillside Zone, which you'll have to then select on the Secret Level Select - in the Secrets menu - and beat. Amy will be waiting at the end, with her being unlocked as a playable character as soon as you touch that sign.