Well, you know how people might only be ticklish in certain areas, sometimes as few as two or three, right? Well, sometimes it seems with people like that, those few areas are super ticklish. I've got a theory that ticklishness flows from the rest of the spots to the ones that are ticklish, which cause that effect.
Where am I going with this? Well, as you may or may not have heard, her feet aren't that ticklish really. However, firsthand experience indicates that that ticklishness has flowed from her feet up to her upperbody, therefore careful application of feathers there should result in a surplus of laughter.
However, such a tickling does not come without a price. Karen seems to be fueled by being tickled, therefore after you tickle her for a long time, she'll be ready to return it. Each and every tickle. Not saying don't do it. Just giving fair warning😉