Naiad

Naiads are guardians and protectors of the waterways, and are bound to one river or stream. Larger river naiads are more powerful, while the naiads of smaller tributaries are less so.
Naiads have a human appearance, but their personality matches their river, i.e strong, lay, hyper, rapid, powerful, meandering, etc. Naiads have total control over their river: they can flood, dry, and manipulate the waters in any way. They can also conjure waterballs in the same way that witches can conjure fireballs.
A naiad can present in either water form or human form. Their water form looks like a liquid human, while their human form is always of someone indigenous to the land they come from.
The only way to kill a naiad is to cut their river off at the source and starve them dry. When a naiad dies, it disintegrates into a puddle of water.
Naiads have a family unit hierarchy. The main river is the matriarch/patriarch, while tributaries are seen as their children.