By "fictional characteristics and qualities," I believe you are looking for animal characteristics that are not real that Kipling has given the snakes.
The first fictional characteristic Kipling gives the cobras is the ability to talk. I'm sure snakes have a way of communicating with each other. I'm also certain other animals can interpret cobras' body language in order to judge their intents, but cobras most definitely don't communicate with each other or other animals in English.
As far as I know, cobras aren't capable of creating elaborate plots to rid their habitat of predators either. Nag and Nagaina decide that, in order to get rid of Rikki-Tikki, it would work best to kill the people who care for him. They believe that, if there are no people, the mongoose will move to different hunting grounds.
"When the house is emptied of people," said Nagaina to her husband, "he will have to go away, and then the garden will be our own again."