I think you can improve the whole character role issue people are bringing up just by doing something with thier portrait backgrounds, rather than a flat color. For example, the scientist could have some basic sciencey stuff in the background, beakers, microscope, jacob's ladder. That sort of thing. The robot can have some circiutry, or electrics of some sort. Just a few minor details to show thier role in the game. I think that would really help convey their identities. If that's not working for you, scietists often wear black rubber gloves. You already have a hand in the view for the robot, shouldn;t be too difficult to add one for your main character. Not sure if it fits with your design, though.
~X