So basically my hero academia and demon slayer are like in the same universe like it's a normal thing they are all at the beach relaxing when you a mermaid come out of the water in your human form in a blue bathing suit you drink some of Minas water and walk away but when Mina looks at her water after trying to stop you there is fish in her water bottle it a clear one they don't know your a mermaid until you go in the water and turn back into a mermaid