Why does the author have Katniss get close to Rue? What purpose does Rue serve for the author?

It’s suppose to show how cruel the games are. Keep in mind that this was an innocent twelve-year-old girl, I repeat twelve, who just died after being brutally speared by Marvel, the boy from District 1. Suzanne makes Katniss get close to Rue for two reasons. 1) because of the obvious resemblance to Prim, reminding Katniss of her little sister and what could’ve happened to her. Having another little twelve-year-old little girl there shows how barbaric the games really was. 2) in many ways, this is the start of the rebellion. Katniss realizes how awful the Capitol really are, which later makes her accept her role as the Mockingjay. She sets the Capitol in a bad light when she gives Rue a «funeral» sort of by decorating her with flowers, which probably didn’t happen much, if it even happened at all. It only highlights how awful the games and the Capitol are. She also promises Rue that she’s going to win, a promise that’s even more important to her than the promise she gave to Prim, that she would return home.

So Rue was a character to show the injustice of the world they lived in and to spark of the rebellion.