Merfolk

Merfolk (魚人族) are a largely amphibious race of Demi-Humans with fairy ancestors similar to dwarves and elves. They can also be called Mermaids or Mermen.

Appearance
So far Merfolk are all scaled humanoids with the heads of fish, webbed fingers, and large fin spines.

Information
Not much is known about the Merfolk besides what was mentioned in Volume 3 of the Light Novels. A community of them resided in Lake Siss though were driven out of their homes due to a 'Growing Horde of Amphibious Monsters' which made them seek protection inside Tempest.