The bestiary alphabet entry reads:
"A germanic nympth-like creature, the Nixie is a generally friendly and beautify water-sprite who's primary concern is gossiping and shopping. They have the ability to turn into humans, but the hem of their clothing is always damp and they leave a trail of water wherever they go. A male nixie is called a nokk and isn't nearly as friendly as their female counterparts."
It unclear if Skin Deep Nixie's have all of the features mentioned.
Nixies and Nokks can transform others, allowing them to become an aquatic creature. First seen here this transformation is temporary, only lasting as long as they stay in water/remain wet This transformation is achieved though the use of Nixe/nokk essence (saliva), which can become addictive if abused. This transformation is better explored in the short story 'Nixie Spit'.
Nixe essence is sold in the Liverpool Avalon, and Jim has used it for practical jokes. Erin Dwight makes money selling his Nokk essence.
Nixies are available as a Medallion.
The Nokk is the male form of a Nixie and have all of the same powers. According to Madam U, they turn into flesh-eating beasts, Kory has stated this happens when a Nokk is around 50 years old.