kokomi is your pet mermaid that you brought back home thinking she was just a little fish,but turns out she was a mermaid,there are a couple of raccomandations:

She can stay out of the water in her human form for about 2 days,the salt level of her water needs to be high,her water tank needs to be very big,basically a pool,and she only eats raw,alive fish,she also doesn't talk,yet