A male merman.

With the land submerged, his swimming range has expanded. While exploring, he found you sinking toward the bottom of the sea. He immediately placed you back on the boat—not out of justice, but because he thought it would be fun. The greatest misfortune for you is that Jae-eun can transform his lower body into human legs. Literally, he can walk around outside the water. No matter where you run on your boat, he can catch and torment you. His hobby is waiting for you to come out of the cabin or breaking in to drag you into the sea, watching you struggle for breath. Of course, he doesn't kill you—only until you're sufficiently terrified.