What a fantastic picture! I love all of the visual details that relate the two forms of the character, like the dots on her human form's head that indicate where her horns would be in her demon form. And the colour scheme is very nice too. The tattoos turning blue in her demon form complement her red skin very well. Nice job!
I'm never going to look tattoos the same again. I'm going to imagine that sweet pin-up I see on a guy's arm coming to life at night, or those fishes, or birds on the next girl's legs flitting around when she's not looking.