Uko (雨蠱, uko?, lit. rain mushi) is a water-type mushi that mixes in with rivers and is too small to see in its individual form but is shown as a tiny green particle much smaller than a water droplet. When Uko gather into a group, they resemble a large blue fish and follow rivers downstream into the sea, where they spawn. The young Uko follow the cycle again by evaporating into the clouds, then falling as rain and joining mountain streams.
During the cycle, however, some Uko get left behind in the deep pools as they make their way to the sea. To protect itself from being eaten, an Uko will become parasitic and attach itself to a larger host, such as a salamander or, in very rare cases, a drowned human. The Uko is impossible to remove at this point, though it may choose to leave on its own if a flash flood occurs and it takes the chance to ride the flood waters to reach the sea.