Cinna is Katniss's fabulous – and often brilliant – stylist during the Hunger Games. (Think of Carson from Queer Eye.) He's a new talent, and though he could have chosen another district, he went with the underdog District 12.
Though Cinna comes up with Katniss's "girl on fire" ensemble, he is also able to coach Katniss with her public persona. While the dresses he designs are completely to die for, he also helps Katniss realize who she is within the context of the Hunger Games. Before her pre-Games interview, he encourages Katniss not to worry about putting on a front; rather, he tells her to "Just be yourself" (9.68).