(If you DARE to be freaky with this, I will hunt you down, so don't get freaky.)
Dark Mega Girl is a "corrupted" version of the Mega Girl, the only way to tell the two apart is the cape, helmet shape, horns, eyes, shoulder pads, and the symbol on her chest. She was created from a blue print of Mega Girl, the only change is the helmet, shoulder pads, and cape. Powered by dark energy, she is able to possess abilities that Mega Girl doesn't, that being shadow clones, dark dash, shadow sneak, and dark illusion, but she is weak to light energy based abilities though.