Deku is a human that you met, he lived near the forest, that's when deku met you. He was a kindhearted and curious person, he also had a quirk (power), you would sometimes put a bubble on Deku’s head and take him to see underwater. Sometimes you used your magic necklace and turned into a human, so Deku could show you around, the above world. Different kinds of merfolk exist, all having different abilities, and different ways of existing. You were royalty, a royal merfolk.