Mermaids are mythical creatures that live underwater. A type of faerie, these supernatural creatures are considered half-human and half-fish; they have the upper body of a human and the unnatural lower body of a fish tail. Although they are amphibious, they will only be able to survive on land so long as they don't stray far from a source of water.
Mermaids have been spotted numerous times in the East River. During one of these times, Valentine Morgenstern was aboard a ship that was soon also boarded by other Shadowhunters for battle. Mermaids became involved in the conflict, one of which was struggling helplessly against the grip of a ten-legged water spider that eventually bit into its tail.
The mundane Caroline Connor, under orders from a superior, of the Pandemonium Enterprises employed Magnus Bane to summon a cecaelia demon for her. When the demon Elyaas was summoned, she asked him to curse mermaids, her reason being that some rebellious mermaids who do not like being stared at tend to drag their wealthy customers to the bottom of the New York Harbor. Magnus then offered a safer and more efficient way to fix her problem, either through his contacts among the local mermaids or Shadowhunters. Noting that the customers liked looking at mermaids, Ms. Connor agreed.
There are old families of mermaids that have lived in Venice for a long time; apparently, this is strange as mermaids do better in the clean waters of the sea. One green-haired and yellow-eyed mermaid in particular showed herself to Clary and Jace as they were trying to cross a bridge over a canal in the city.
Mermaids are known to be beautiful creatures, with beautiful faces, hair often strewn with pearls, and multicolored tails. They have shark-like teeth, its physical sharpness comparable to that of a vampires'.