Nina Myers was an agent at the Los Angeles Counter-Terrorist Unit, who turned out to be a ruthless sociopath who was selling information to various terrorist organizations. She used to be romantically involved with Jack Bauer when he was separated from his wife Teri, but Jack broke it off when he and Teri got back together. When Day One started, Nina was dating Tony Almeida.
Nina was second-in-command of CTU, and seemingly very loyal to Jack, breaking many of CTU's rules and protocols ot aid him. But unknown to anyone, she was secretly selling information to the Drazens. She secretly interfered in investigations and was aiding in the attempts on Presidential candidate David Palmer's life. She murdered a second mole, Jamey and erased some of the camera footage showing her committing the murder. She provided the location of a safe house housing Jack's family to the Drazen cell. After the safe house was attacked, she encouraged George Mason to tell Jack about it, which would hopefully distract him. She also was ordered by the Drazens to tell Jack that his daughter was dead, in an effort to make him attack them in a blind rage. She did so, knowing that if Jack lived that her cover would be blown. During her escape from CTU, Teri Bauer became aware that Nina was a traitor. She overheard part of Nina's escape plan, so Nina killed her. She was captured by Jack when attempting to escape CTU. Jack nearly killed her then and there, but Mason talked him into letting her be taken into custody. It was only later that Jack discovered that his wife was dead.
Now imprisoned, evidence shows up linking Nina the Wald Cell of that day's terrorist attack. That information allowed the Wald Cell to plant bombs in CTU, killing many people, some of which were former friends of Nina. In exchange for Nina's help in stopping the threat of the nuclear bomb, she is promised a Presidential pardon. Nina took them to a terrorist hideout in San Diego where she attempted and failed to escape, but they captured a suspect. They were flying back to Los Angeles with Nina acting as translator. Nina got the suspect to give her the information, and then she slit his throat with a sharp object she picked up in the San Diego hideout. Shortly afterard, their plane was shot down by the Stanton paramilitary cell. Nina wasn't breathing after the crash, but Jack revived her. Then they were attacked by the Stanton Paramilitary cell and Nina assisted Jack for their mutual survival. After the crisis was over, Nina fearing that Jack will kill her, got the drop on Jack. She captured Jack demanded an additional Presidential pardon specifically for the murder of Jack Bauer as part of her conditions. Palmer agrees, but Nina is recaptured shortly thereafter and never got the chance to kill Jack. Palmer gave her the original pardon, but tore up the pardon allowing Jack's murder.
Nina is encountered in Mexico while trying to purchase a biological weapon from arms dealers. She is captured and brought back to Los Angeles CTU, where she attempts suicide, then when medical care is being performed on her, she escapes and is killed by Jack Bauer when she tries to draw a weapon on him.