Kappa are creatures from Japan said to lurk in rivers and lakes. They are described as small humanoids resembling a cross between a monkey, a frog, and a turtle with either a water filled depression or a patch of water soaked flesh atop its head (which is supposedly the source of its strength) surrounded by a ring of hair. It’s said to have a penchant for drowning people and eating their intestines, has a fondness for cucumbers and sumo wrestling, and the ability to fire off high powered farts at will. The question is whether they could evolve in nature.
They could be large salamanders, or something similar, that never lost their feathery gills, but adapted to come on land despite this. The bowl could be a depression on the head to keep the gills moist, while the gills themselves could become even more feather like as a second method of holding water, like how hair can stay wet for hours, and this could give the appearance of hair surrounding the bowl as some of the extended gills might flop out the edge of the bowl a bit. It could have a thick temnospondyl-type body to give the vague appearance of a turtle, while it's forelimbs could be spindly grasping ordeals for the monkey comparison. Big meat eating amphibians might supplement it's diet with cucumbers, and people for that matter, though mostly children due to the threatening size of adults, but never underestimate a starving endangered animal. The enjoyment of sumo wrestling might just be an extension of an adaptation for waiting out fights between other species with the intent of drowning the weakened loser. This obviously won't be possible with the Sumo wrestler, the adaptation is there.
For evolution it makes sense to start out with an aquatic salamander, like the axolotl. They require water and have arms and legs. Lengthening them and growing larger could help them to catch land animals and drown them to eat them, and with sufficient size and strength humans could be on the menue as well.
Various amphibious creatures that have gills can survive on land for a few hours if they keep their gills wet (crabs are common examples). This could not really become a water-filled depression, as that wouldn't make much sense for hunting outside of water, but it would be the closest to a water-filled sponge. As omnivores, they probably could have dietary reasons to eat cucumbers and intestines (carnivores eat intestines because they can't process a lot of plant-based food, they rely on their quarry to, umm, pre-process some of it).
Most of it yes but the water filled depression in the head no. You need a reason why evolution would favor a water bowl for a head over not having a bowl and there is no reason for and lots of reason for why not.
I'm also not sure but I don't think there is a genetic component to enjoying watching two fat guys wrestle.