It is now known that Blues is unlockable after completing all 100 challenges in the game. He's weaker than usual (apparently since he's a prototype robot), but he makes up for it by firing strong shots without charging, jumping higher, and deflecting enemy shots with his shield whenver he isn't moving. Here's some screenshots of Proto Man in action at the game's intro stage:
Pics
And here's a couple more shots of the intro stage (please excuse the small size, they're from streamed GameSpot videos):
Rock jumping around. You start the game out like this, and you can't play as him again until you beat the entire game as Rockman in Hard mode. His only attack is a kick (think MM8's Mega Ball, but without the ball itself).
Fire Man about to fight the boss of the stage, which is a giant version of those big one-eyed jumping robots from the original game.