This has been keeping me up at night.
The note on the artwork here says (シール). This can mean both seal, as in demonic seal, or sticker.
Have I been mistaken? Has her thigh tattoo always been just a children's sticker? Not a physical manifestation of her powerful magical energy, her deep connection to her nature as a devil? Merely a temporary decoration, easily washed off with a bit of water and soap?

I won't accept this. It was real in my mind. It's a demonic seal for sure!