It is said that kappa range from the mischievous to the downright evil. They play pranks and murder people, depending on their whim at the time. Their one weakness is a bowl that they keep on (or, in some legends, in) their heads. In this bowl, they store water, which is necessary for movement and life. Should you encounter one, the best method of defense is to be exceedingly polite and exchange a series of bows. Eventually, the kappa will bow low enough that their water spills out and they become frozen in place. At this point you may flee or, if you wish, refill the bowl from a nearby stream, after which the kappa will become your servant.
Over the years, kappa have shed their legs, evolving to protect themselves against the bowing attack. Modern day kappa no longer have the ability to bow and view excessive politeness negatively as it is quite rude to expect them to bow when they cannot. They are, however, still vulnerable to dehydration, as shown here.