Kappa

In Medieval Japan, a breed of humanoid creature called the Kappa was believed at the time to dwell in rivers and swampy areas.
It was a type of vampirelike lecherous creature that is more intelligent than the devilish, and less malevolent toward men.

They walked erect, though their extremities ended in webbed structures that looked not unlike claws. There hung down from their large mo­bile ears weird appendages resembling long narrow earrings. Their eyes were triangular and elongated, while on the top of the head there sat what appeared to some observers to be a bald spot, to others a big ball of yarn out of which stuck four lengthy darning needles.
Whatever the case may be, legends have grown up around them suggesting that they are dirty; greedy and lazy-very much like some of the "wee folk" of the Celtic legends.

They are, however, extremely polite. So here is what you do: you will recognize a Kappa from the bowl-like depression on top of its head filled with water. This is its power source.

So you bow to a KAPPA in the approved Japanese fashion, and naturally having to return the courtesy it will bow back and the water tips out. It is then powerless until the water can be replenished.

This is not all. They are crazy about cucumbers. They prefer cucumbers to blood. Bribe them with a cucumber and they will promise you almost anything. And once a promise is made they are honor-bound to keep it.