The Mermaids
Okay, mermaids, pretty basic, right? Well...There are some things that should be pointed out.
All mermaids are female. They do wear tops, whether they're seashells like Ariel wears in The Little Mermaid (Although I'm not sure how you would get them to stay) or halter tops, or whatever.
Most mermaids are named after gems of some sort, or names that have meanings associated with gems. They will also choose names that have water associations, (eg. Water Lily)
They also have the ability to separate their fins into legs, and go on land for short periods of time. Most of them prefer to stay in the water, unless they're having fun with some handsome sea elf. When they do come ashore, they can only stay for a few days, before the call of the ocean lures them back.
Mermaids love pretty baubles, and such, and often the man that one chooses is the one that brought her the prettiest things.
They become human to give birth, and any boy children are left at Crystal Cove to be found and raised by the caretakers, a group of women dedicated to raising the boys. This is not because mermaids can't be bothered to care for their offspring, but rather the boy children can't breathe underwater as their mothers and female counterparts can, and therefore the mermaids can't take care of them.
The mermaids are ruled by a Queen Nirvelli, who lives in the crystal palace deep beneath the waters. She is one of the few who have Joined with the water dragons, a huge creature that has the ability to breathe under water and can also fly, like it's land-based cousins, and is the only mermaid Tahani.
I'll stick the water dragons in here, as they go together with the mermaids.
Water dragons are far more streamlined then their cousins. Their bodies are slimmer, and their legs are shorter, designed to be pulled close to the body for fast swimming. The wing tips, when folded, are covered by a thin flap of skin that reduces drag. water dragons have very little heirarchy, as they are normally solitary creatures, coming together only to breed and raise the young. The females are always a bluish colour, and the males are green, though they can be any shades, from the exceedingly rare silver-blue of Nirvelli's Misu, to the equally hard-to-find near-black green.