A child-sized amphibious creature with a turtle's shell, a beaked face, and a dish-like depression on its crown that must stay filled with water — the source of its unnatural strength. Kappa folklore is unusually consistent across Japan and unusually practical: it exists substantially to keep children away from rivers.
The kappa is dangerous — it drags swimmers under — but it is also compulsively courteous and scrupulously honest, which is its documented weakness. Bow deeply to a kappa and it must bow back, spilling its crown water and its strength with it. A kappa whose water you refill from your own canteen is bound to you in gratitude, and kappa debts are said to be repaid across generations.
It can also be bought off outright: cucumbers, its favorite food above all things, thrown into the river with the family's names carved in — an offering some riverside neighborhoods maintained within living memory.