I managed to get to level 5 or 6, unfortunately Rosa couldn't stop the red monster, even though she was directly in front of him.
Moving characters should definitely be relegated to WASD and with a right mouse click to switch between characters and left mouse click to attack.
Your first bullet is invisible and you can see what you're shooting when it's already shot. I finished couple of early levels without understanding what I'm doing.
Other than that I actually like how the characters look, but everything is pretty inconsistent.