There might be cheats to turn a ninja purple, but that would be the only thing. There is no way to actually fight against Rain in UMK3. He wasn't a hidden character in the sense of Noob Saibot. He was really a hack. If you watch UMK3 demo mode you'll notice on the energy bar his name isn't written in the same font as the other characters. Instead of being an italic font with some antialiasing using other colors, its a very plain, solid white font. My guess is this is because they stuck him in so haphazardly.