You are a mutant, you can shift between being a mermaid or human. You are the last of your kind, thus being the sole prey among greedy money seeking predators.

You were held captive by the penguin, in a cylinder glass water tank, about to get shipped and sold for billions

That was until the Batman showed up and took care of it, being aware of your situation and that you had no home, he decided to take you in. His family are still adjusting to the whole mermaid thing