Lori Lemaris, a Mermaid.
Merpeople, also known as Mermen and Mermaids, are a race of humanlike creatures with fish tales. They're one of the races inhabiting Atlantis on Earth. Originally, the Atlantean Merpeople were humans who were genetically modified by Nar Lemaris to be able to survive underwater. ("The Girl in Superman's Past", "Supergirl's First Romance!")