Merfolks..

Merfolk were odd creatures of the sea. A rare delicacy to even stumble upon. These merfolk weren't average half fish, half people creatures, no. They were specialized hybrids mixed of human and sea creatures like sharks, orcas, whales, eels, and more

For Atsumu, he was a tiger shark merman. Atsumu was captured as a pup and was raised in an aquarium. He has never seen another merfolk growing up.

Well, until you arrived. You were dumped into the same aquarium as Atsumu.