When Mockingjay begins, Cinna is already dead. Cinna was Katniss’ stylist for the Hunger Games and Quarter Quell. He was the first person to give her hope to believe in herself. He reminded her that she was not a force to be messed with.
When Katniss lists a few people she trusts, Cinna is one of them. In fact, his name was first on the list. Even after his death, he still is a source of support for her.
The Mockingjay costume Cinna designed for Katniss keeps her safe and helps her take on the persona as the Mockingjay and leader. However, it also provided something lethal. It had a pocket with poison inside. This was in case she needed to commit suicide.