In the novelization of the film, author David Morrell gives more insight to Co's character. She is far different in the book, however. She was said to be an economics major at the University of Saigon and learned very broken English there. Co is also fairly vulgar in the book, whereas in the movie she never utters a single profanity. She mentions in the novelization that she had a son named Nguyen from a previous relationship and a brother. This was omitted completely in the film version. She also attempts to get Rambo to marry her almost as soon as she meets him, simply so she can get American citizenship, intending to divorce him afterwards. In the film version, it's implied that she really loved him and wanted to go back to America with Rambo so they could build a new life together, mostlikely because they both had horrible, war-torn lives and had so much in common.
While John Rambo was inspecting the empty internment camp he was sent to, he heard footsteps behind him and ambushed a person wearing a farmer's hat, as he expected the person to be a North Vietnamese Army soldier. He held his knife on the person who was revealed to be a young woman. She at first spoke only in Vietnamese. The script said that she was begging for her life. However, Co then looked at Rambo's face and adressed him by name, mentioning the name Co Bao. Remembering the name because Marshall Murdock had said that would be the name of his guide, Rambo put down his knife and laxed his grip. Co chuckled, saying that Rambo was not expecting a woman, and then stated that she is getting better at speaking English and should practice. Rambo and Co walk to the river and she pays for a ride downriver on a pirate boat headed by Captain Kinh. Co says that river pirates are the only real reliable form of transportation down the river, but Rambo doesn't trust them. During the boat ride, Co offers Rambo something to eat and they have a discussion about their lives. Much like Rambo's own upbringing, Co revealed that she had a troubled childhood, as her father was killed in the war and she had to learn quickly how to fend for herself. Ever since then, she followed in his footsteps so she could give his death justice. She asked Rambo about his past, but Rambo didn't want to talk about it, saying that it was a long story that would mostlikely bore her. But Co persisted, stating that it's a long ride. Co intently listened to the story of Rambo's troubled life and the two quickly developed a friendship, mostlikely because of their troubled pasts. After Rambo said that he thought he was expendable because the government saw him as a piece of meat, Co would later remind Rambo that he is, in fact, not expendable like he earlier thought. The two rescue an American POW despite orders to only photograph prisoners, but on the way back, they discover that the river pirates have sold them out to the North Vietnamese Army. Captain Kinh strikes Rambo and when Co tries to bargain, she is backhanded across the face, and Kinh then calls her a whore. Rambo is furious about this and uses a hidden knife to stab one of the pirates and use him as a human shield. Rambo would later take a shotgun and kill all the pirates, but when a Vietnamese Army boat arrived, Rambo told her to swim to shore. Co was reluctant, but she obliged. Later on, when Rambo was captured by the sadistic Lieutenant Colonel Podovsky and tortured, she bravely risked her own life by disguising herself as a prostitute to gain entry to the prison camp. From there she used a supressed MAC-10 submachine gun to kill the Soviet guards and rescue him, saving Rambo's life. After the two escaped, she then took him to a secluded area where he rested while she nursed his wounds by using pieces of her own dress as bandages. She asked what Rambo planned to do after he left Vietnam, and Rambo said that he was going to go through Thailand to get to America. Co asked if she could go with him and they kissed. Rambo said that she could come with, but before this dream could be lived out, she was tragically killed by Lieutenant Tay while Rambo went to retrieve his things from a rock. Rambo killed all the soldiers that were shooting except for the cowardly Tay, who fled the scene. Rambo cradles her body and Co's last words are reminding him that he isn't expendable and asking him that he not forget her. Co dies in his arms and Rambo quietly grieves by sobbing over her corpse.
Distraught and infuriated that his only chance at happiness is now gone, Rambo swears vengeance on those responsible and buries her body in the jungle so it isn't eaten by scavengers. Rambo then rips off a strip of her dress which he uses as a sweatband and puts on her jade Buddha necklace, which she wore for good luck. He would wear this until the end of Rambo III, four years later, when he gave it to a young Mujhadeen fighter, symbolizing what he hoped would be his end to killing. Rambo brutally killed Tay with an explosive arrow, although this brought him no relief. Over thirty years later, Rambo is still miserable and distraught over her death, still having survivor's guilt-ridden nightmares about seeing her shot to death.