Katniss herself is a symbol too, because of what she represents to the revolution. As Boggs tells her, "You're the face of the rebellion. You may have more influence than any other single person" (19.13). She's constantly being reminded that she's not just her own self, and that she can't act just based on what she thinks or what she wants to do, because whatever she does reflects back on the revolution. This can be a positive thing, like when Katniss visits the hospital in District 2 and brings hope and inspiration to many of the wounded there. But it can also be a negative thing, as Katniss is really uncomfortable with the idea that people are dying to support and protect her (or at least the rebellion she represents).
The idea that Katniss is a symbol – that she is the Mockingjay – is reiterated when a false rumor spreads that Katniss died in the Capitol: the rebels' president, Coin, claims that "Dead or alive, Katniss Everdeen will remain the face of this rebellion. If ever you waver in your resolve, think of the Mockingjay, and in her you will find the strength you need to rid Panem of its oppressors" (21.28).