You remember Aesop, right? He's the Greek slave who is credited with creating a whole bunch of fables, stories that usually contain animal characters and portray particular lessons or moral teachings. Well, Kipling took a page out of Aesop's playbook when writing "Rikki-Tikki-Tavi" because the tone is totally fabulist. (LOL. Sorry.)
Check out the moralist tone:
That night at dinner, walking to and fro among the wineglasses on the table, [Rikki-tikki] could have stuffed himself three times over with nice things; but he remembered Nag and Nagaina […]. (38).
Although Rikki-tikki could indulge in the finer things in life, he chooses not to because they'll only slow him down in the battles to come. The tone of that passage slyly suggests that this anti-gluttony attitude is one the reader should also take up when necessary. In fact, many aspects of Rikki-tikki's character, from his courage to his curiosity, are held up by the novel's tone as characteristics that its readers—originally young boys—should emulate.
On the flip side are characters like "feather-brained" (78) Darzee. Rikki-tikki's complaint that Darzee doesn't "know when to do the right thing at the right time" is aimed as much at the reader as it is at the bird (69).