Katniss represents hope to her people, and a symbol of revolution.
Katniss did not ask to be a revolutionary or a martyr. She participated in the Hunger Games to save her young sister, who was too weak to participate. Yet Katniss was a part of the rebellion long before she even realized it. She won the hunger games, and threatened to commit suicide if the rules did not allow two winners. She also wore the mockingjay pin, given to her as a gift, without knowing what it meant.
When things reach a head with the revolution, Katniss realizes it does not matter what she has done in the past. She has to be more than a symbol now.
I must now become the actual leader, the face, the voice, the embodiment of the revolution. (p. 10)
The ironic thing is that the actual leaders of the rebellion do not really want her to be anything other than a symbol. As a symbol, she is a rallying point but relatively harmless. As a leader, she may have something to say.
Haymitch reinforces for Katniss her real role in the revolution. When Katniss tries to make a propaganda video, she does not do a very good job.
Finally, the intercom crackles and Haymitch’s acerbic laugh fills the studio. He contains himself just long enough to say, “And that, my friends, is how a revolution dies.” (p. 72)
From this point on, Katniss becomes more involved. She actually goes out into battle, not as a photo opportunity but as a fighter. She begins to wonder more and more about what District 13 and the leaders are telling her.
Eventually, she comes to the realization that she cannot trust them. She asks to be the one to kill Snow, but she knows that killing him will not solve the people’s problems in Panem, because the leaders of the rebellion are just as ruthless. They want to hold a Hunger Games with capital children, for one thing. When they set off the bomb that kills many innocent victims in the capital, including her sister, Katniss acts.
Coin collapses over the side of the balcony and plunges to the ground. Dead. (p. 372)
Katniss chooses to kill Coin instead of Snow, in order to end the cycle of violence. In this way, she becomes not just a symbol of the rebellion, but a real participant. She ends the war and violence once and for all.
Collins, Suzanne (2010-08-24). Mockingjay (The Final Book of The Hunger Games). Scholastic Books. Kindle Edition.