"Brilliant, enigmatic, lovely Cinna" (1.34) is already dead when the book begins. He's one of the first people to die for Katniss, and by no means is he the last. Cinna was her stylist for the Games and the Quell. He dressed her and gave her the first feeling that she could believe in herself – that she was a force to be reckoned with. He wasn't just a clothes designer; by creating fabulous outfits for Katniss, Cinna helped further the revolution's cause.
When Katniss mentions the shortlist of people who she trusts or used to trust, Cinna's name is right at the top. And he continues to be a source of support and help after his death. His costumes make her into the Mockingjay and they also keep her safe: they help her take on the persona of the Mockingjay and be a leader, but they also provide for the worst, by containing a pocket for poison in case she needs to commit suicide.