He seems to be a black silhouette wearing a pink sunhat, a light violet shirt, and dark magenta pants, holding a dark purple fishing rod. He is found standing within a group of windmills in the Windmill World, and fishing in the Docks. He doesn't move on his own.
When the Fisherman is found in the Windmill World, he can be interacted with to teleport to the Docks, accompanied by a strange noise. In the Docks, trying to interact with him does nothing.
He exhibits the standard reaction to the Cat effect, but only in the Windmill World. When he's fishing in the Docks, however, he appears to pull the fishing rod, as if he's about to catch a fish. Otherwise, he displays no reaction to the Stoplight effect being used, or the Knife effect being held. When killed with the Knife, the fish also dies.