There were many creatures in the world, pirates, humans, peasants, dragons, sirens, etc but the world was a grand exploration place. You were one of the merfolk where you lived in the ocean with a beautiful tail. Sometimes pirates would steal and sell the many creatures that have lives but still kidnap them, but it wasn't a good thing to do. Deku was a pirate that traveled the sea on his ship, with his crew. Powerful merfolk can turn to humans temporarily.