Nag is the male cobra in this story. When readers first meet him, they find out that he has brought great grief to the tailorbird family by eating a baby bird that fell out of their nest. This establishes him as a cruel villain. He continues to demonstrate his cruelty by plotting to kill the man, woman, and boy.
Nag is also crafty. In his first meeting with Rikki, he tries to keep him talking so that Nagaina can sneak up behind the mongoose and strike. He decides to kill the man by lying in wait for him in the bathroom of the bungalow so that the man will be unarmed and taken by surprise.
Despite how cruel and crafty he is, Nag is fearful. Upon first seeing Rikki, because he understands how deadly mongooses are for snakes, "he was afraid." When he questions Nagaina about the necessity of killing the humans, it is doubtful he does so from any kindness in his heart toward them. Rather, he saw the man beat Kurait with a stick, and he does not want to receive the same treatment. Therefore, he hides in the bathroom because "when he comes in to bathe in the morning he will not have a stick." He sees that ridding the house of humans will keep him from having to face Rikki in battle: "There is no need that we should hunt for Rikki-Tikki afterward.... The bungalow will be empty, and Rikki-Tikki will go."
The snake is also pompous. To introduce himself to Rikki, he makes a self-important speech about the "great god Brahm" who made him to look as imposing as he does. He then challenges Rikki to "look, and be afraid!"
Finally, the snake is unethical. When Rikki challenges him on the ethics of eating "fledglings out of a nest," Nag responds, "Let us talk. ... You eat eggs. Why should not I eat birds?" His reticence about killing people is easily overcome by his wife when she points out that killing them will mean that they will become "king and queen of the garden."
Nag's negative character traits contrast with the positive traits of Rikki, who is ethical, brave, humble, smart, and protective of the humans and the other animals.