Darzee and his wife are tailorbirds. In the story "Rikki-Tikki-Tavi," Rudyard Kipling uses these two characters as a foil and a sidekick for Rikki, the mongoose.
Darzee is a foil. A foil is a character who reflects the qualities of another character in the story through contrast. One of Rikki's outstanding qualities is bravery; Darzee, on the other hand, is timid. When Rikki asks who Nag is, he cowers down in his nest. Rikki is also very intelligent and forward-thinking; Darzee "could never hold more than one idea at a time in his head." These contrasts with Rikki make the mongoose seem all the more heroic.
Darzee's wife is a sidekick. A sidekick is a character who assists the hero. Granted, she only helps him at one point in the story and doesn't accompany him on most of his adventures. But in the scene where Rikki finds and destroys Nagaina's eggs, Darzee's wife is a fine assistant. Pretending to have a broken wing, she distracts Nagaina long enough to let Rikki locate and crack open the eggs and kill the baby snakes. She does this at great personal risk. She then calls out to Rikki to warn him that Nagaina has gone to the bungalow. Most heroes have a sidekick, and Darzee's wife fulfills that role for Rikki when he needs it.
Both Darzee as a foil and Darzee's wife as a sidekick help portray Rikki as a brave and cunning hero.