Art theft is always terrible, actual artists work hard to make a "final" product, and have a hard time actually making more than about 50c an hour for their efforts because people think it is too much for "just a picture".
I know how it feels myself, my own flesh and blood wanted to use my original, hand drawn, approximately 30 hour to ink drawing and claim it as her own for a university course, of course I told her where to go. She was uphill drawing a stick figure, how she thought she would get away with claiming some of my artwork as her own I do not know.
I would be absolutely gutted if someone was using my persona as their own too, I don't mind if someone has something similar, there are only so many colour schemes and the like out their that actually look good, but to just *take* it and use it as your own? No.