You may be looking for the Mermaid from the Harry Potter theme or the Mermaids from the Pirates of the Caribbean Theme.
Mermaid is a Pirates minifigure released in 2009. She appears in two sets, 6243 Brickbeard's Bounty and 6299 Pirates Advent Calendar. She also appears in the Concept Art for LEGO Battles, but never actually appeared in the videogame itself. The tail piece first appeared as part of the Harry Potter Mermaid in 4762 Rescue from the Merpeople. She wasnt really a character she was a statue.
Concept Art for LEGO Battles, with an Islander