The District 6 female tribute participated in a previous game and won. After her Games, she turned to a drug called morphling to be relieved of the painful memories. She is called a "Morphling" because she used the drug so much that she became addicted to it. Her district partner was also a morphling. At an unknown point, Haymitch Abernathy took her and her district partner aside and informed them of a plan to get Katniss Everdeen and Peeta Mellark out of the arena.
75th Hunger Games
During the training sessions, the District 6 tributes were seen at the painting table practicing camouflage. The two tributes painted each other and were shown to be mentally unstable. During her private session with the Gamemakers, the female tribute showed them her camouflage skills.
At the Cornucopia, it is most likely she did not participate in the bloodbath and just ran away from the terror. In the film, when Katniss is running on one of the spokes, she looks to her left and you can see the morphling running into the jungle. She managed to survive whilst her district counterpart did not.
During the orange monkey attack, Peeta was distracted by Katniss. A monkey was about to kill him when the Morphling jumped in the way to protect Peeta. After Peeta had fought off the monkey, he took her body to the beach and held her. The Morphling took some of her blood and drew a flower on Peeta's cheek, which he commented on, saying that it was lovely. The Morphling smiled and soon after, bled to death. Peeta took her corpse to the water and let it drift off until a hovercraft came to take it away. In the film, she dies a similar way, but Peeta tells her to focus on the colors in the sky, giving her a sense of peace, as she bleeds to death in his arms. He then lets her body float and Katniss and him watch as a hovercraft lifts her corpse.