Is there really a mermaid in endless ocean?

Unfortunatly not. :(

Its explained as if there is when you adventure into the 'Mermaids Grotto' where you would expect to see one. But actually, (after searching hard!), you will find a cavern in which there is a West Indian Manatee (or 2!). Catherine explains these as mermaids and that's really just what you have to believe!