Kappa are devious creatures. They linger just below the surface of rivers and lakes, waiting for mortals to test the water before snatching and drowning their fleshy meal. Taken merely at their appearance and juvenile sense of humor, they could be mistaken for mischievous turtle-men, almost playful in a rude sort of way, but they are dangerously strong and ruthless when submerging their prey. King among these monsters is Kuzenbo; bigger, stronger, and more cunning than the rest, but just as infantile.
Kuzenbo led his nine thousand Kappa followers to Japan, where they harried river villages and fishing boats, using their unnatural strength to pull mortals below. So clever, so hungry had the Kappa become, that Japan's army marched to fight back, but they were no match.
As the bowl-shaped indentation atop a Kappa's head is filled with water, that Kappa grows impossibly stronger - far stronger than a mortal. Yet, bound by their strange sense of honor, a Kappa must bow when bowed to, spilling their power upon the ground. It is the only known way to defeat one.
And of the countless battles Kuzenbo has fought, he was defeated only twice. Upon his second defeat at the hands of General Kiyomasa, Kuzenbo was forced to swear to never again sow strife between man and beast, a vow the Kappa King has honored ever since.
Gods, however, are fair game.