Cinna's special in ways that have nothing to do with being played by a rock star. His first words to Katniss are "I'm sorry this happened to you," the first time anyone from the Capitol really acknowledges the terrible fate that will almost certainly befall her.
Unlike Effie and Haymitch, who are wrapped up in their own little worlds, Cinna is on Katniss's side from the beginning. He goes above and beyond to help her feel like she has a friend in this upside-down world.
Cinna's a costume designer, and he handles everything associated with Katniss's look. He's the one who comes up with the dress that seems to burn, which helps give her the catchy nickname "The Girl on Fire." He also helps show Katniss how appearance and posturing are very important in this world, and that he does what he does for her benefit.
CINNA: Did they explain about trying to get sponsors?
KATNISS: Yeah, but I'm not very good at making friends.
CINNA: We'll see.
It starts with his tone, which is kind and gentle no matter what's going on. He makes it very clear that he's here to help Katniss. Unlike Effie or Haymitch, he doesn't treat her like some rube out of the sticks. He explains things patiently to her as a smart woman in a new environment who can pick up on the game with enough of a road map:
KATNISS: So you're here to make me look pretty.
CINNA: I'm here to help you make an impression.
He's good with the flash and sizzle, and in those early days before the actual bloodletting starts, she needs his expertise on her side.
But beyond the good advice, there's the sense that he genuinely likes Katniss. Effie treats her like a problem to be managed, while Haymitch clearly wants to avoid getting too close before her almost certain death in the arena. But Cinna? He treats her like someone he'd like to hang out and play music with were they to meet in some other circumstances. That gives him one last little boost to send her way: a little vote of confidence before she steps into the arena.
CINNA: I'm not allowed to bet, but if I could, I'd bet on you.
That's a true bud, that is. As if he weren't awesome enough, he somehow got his hands on that mockingjay pin that Prim gave to her sister when she volunteered for the Games. He gives it to Katniss right before she gets sent off to the kill zone. Political revolutions get started with such gestures…but that's getting ahead of ourselves. For now, it's enough to know that this guy really cares about her.
And even better, he becomes a mentor as cool as… well, as cool as a rock star showing up for a key part in the movie version of your favorite book.