Kappa are a type of yokai that inhabit the swamps, lakes and rivers of Nippon. They look like small humanoids with reptilian features and are about the size of a child. Their face looks like a turtle and they have a shell on their back. Its scaly skin varies in color from green to to blue. Kappas are both hated and adored by humans because, like all yokai, they are arrogant and contemptuous of humans. However, kappas have the arcane ability to turn the waters they inhabit into healing waters, and many humans have been cured of fatal injuries or diseases considered untreatable in other parts of the world by drinking from these waters. This miraculous ability makes humans tolerate the constant taunts and tricks that kappa like to play on mortals. When areas near rivers and lakes are threatened by war, it is not uncommon for kappa to join armies as skirmish bands, firing poisoned darts at enemies that threaten their home.
You can learn more about the Nippon the Oni shogunate fan project here: https://www.worldanvil.com/w/warhammer-fan-concept3A-nippon-the-oni-shogunate-danilagi7