Lori Lemaris is a mermaid from Tritonis, a city in the undersea lost continent of Atlantis in the DC universe. She was also one of Superman's many loves. We are first introduced to Lori in Superman #129 volume 1 in May 1959.
Basically she was pretending to be human, hiding her tail with a blanket and sitting in a wheelchair. Clark Kent (Superman) falls for her but after a short blissful time together she turns down his proposal. Lori tells him she must return to her homeland. Clark gets suspicious and finds out her secret, she's a mermaid! They remain friends and we see her show up occasionally in Superman comics. At some point Superman became a merman himself to be with Lori, that or Lois had a disturbing fantasy about it happening (for a chick who didn't know Supes secret identity she sure was jealous.)