If this was meant to be a joke, I say it's a great joke! I can only imagine how many people (like myself) fell for it.
On the other hand, if I were to draw her, I would probably draw her feet and any other area that's being tickled but nothing else. Explanation? Any excuse you want... you could say she looses her invisibility only when being tickled, and only on those areas... whatever you wish to say.
But dude, seriously... draw something, at least her face!!