I think it's because she is probably one of the first (if not THE first) super hot girl that's got the hots for bad dudes. which pretty much makes her unattainable, and everyone wants what they can't have
Have you noticed how they are complete opposites, and yet they look good together? Creepy scary and dark Hades, pretty and light and adorable Persephone! I love opposites in couples. XD Ah well, you captured it well!