How to create a mermaid character?

So, I'm escritura a story that is set in Pre-Islamic Middle East, namely the area of Lebanon and Jordan. And one of the characters is a mermaid (I know, it's weird) and I have her name planned and all but I don't know how to make her less modern and tame. I would appreciate your help.
This story is fantasía (obviously) and rather unrealistic in some ways. If tu would like más information on the story just let me know.