How do you say "the real mermaid" in French?

I'm creating a Twitter username; unfortunately "therealmermaid" has been taken by a name waster. Therefore, I would like to use it in French. I know how to say Mermaid ("Sirene"), but I'm not sure what format "the" should be in and I don't know how you say "real". Basically, I just want to know how to say "the real mermaid" in French. I hope someone can help, thanks!