Cressida is in charge of the media team and camera crew that film and follow Katniss around "in action." While at first it seems like Cressida is a skilled manipulator along with others like Plutarch and even Coin or Snow, later we see her using her potential for manipulation and quick thinking to support Katniss when Squad 451 is in the field. She comes up with a reason to support Katniss's bogus secret mission.
Ultimately, it looks as though Cressida makes it through the war alive. She's one of the few Squad members to make it through the underground sections of the Capitol and into the (almost as) dangerous streets. It's also down to Cressida that Katniss and the other Squad members have anywhere to go at all once they emerge from the underground, since it's her contact, Tigris, who offers the rebels a hiding place.