Cinna has a small but important role in Catching Fire. He's one of the people Katniss trusts most in the world, and he's also as one of the bravest people in Panem. You might think that as a clothing designer Cinna can't really contribute to a potential revolution or make politically charged statements. But in fact, Cinna helps fuel the fires of the rebellion by creating Katniss' image as an iconic figure. When he comes up with a new look for Katniss to premiere in the Games' opening ceremonies, Katniss is amazed:
He turns me toward a mirror so that I can take in the entire effect. I do not see a girl, or even a woman, but some unearthly being who looks like she might make her home in the volcano that destroyed so many in Haymitch's Quell. The black crown, which now appears red-hot, casts strange shadows on my dramatically made-up face. Katniss, the girl on fire, has left behind her flickering flames and bejeweled gowns and soft candlelight frocks. She is as deadly as fire itself. (15.15)
Cinna is great at makeovers, and this one's not your garden-variety episode of What Not to Wear. Katniss' new look has to show how much she's changed – she's no longer a girl wearing "flickering flames and bejeweled gowns and soft candlelight frocks," but has become a dangerous force to be reckoned with, one that "is as deadly as fire itself." Cinna has helped her appear as the best version of herself, groomed her so that her outside is as dangerous and beautiful as her inside. Without him, she probably wouldn't have had as much impact on the crowds in either of her Hunger Games appearances.
Also, did we mention how brave Cinna is? Think about the wedding dress he designs for Katniss, which is like a declaration of revolution itself. It looks just like her Capitol-approved wedding dress, but it has a secret explosion built in. When Katniss moves in a certain way, the dress catches on fire and transforms from a pure white wedding dress to a dark mockingjay costume.
This transformation is Cinna's commentary on several points: the rejection of the president and the people's choice (they voted on Katniss' wedding gown), the devastation of a wedding that is not to be, the power of Katniss as the mockingjay, and the creation of a potent symbol that underscores the claims of the revolution. Of course, he will be punished for it, but first there's applause:
Caesar gestures for Cinna to rise. He does, and makes a small, gracious bow. And suddenly I am so afraid for him. What has he done? Something terribly dangerous. An act of rebellion in itself. And he's done it for me. (18.4)
Katniss is right to be afraid. She will have to stand by and watch helplessly as Cinna is beaten to death in front of her, all because he was brave enough to help her and broadcast his rebellious message to the people of Panem.