Tara Duncan

A mermaid named Lorelei poisons Tara to turn her into a mermaid. Lorelei will only give the antidote on the condition that her sister, Odarelle, is released from the Otherworld prison. There's a reason that Odarelle is in prison, though. With her sister, Odarelle has the ability to flood the earth and Otherworld with her song. However, Cal appears to actually know the enemy on a more personal level.