Born to an unknown mother and father and the niece of former SECNAV Phillip Davenport, EJ grew up in an unknown area where she spent some time as a cheerleader during high school.
After graduating from high school and presumably from college, EJ joined NCIS with her advancement within the agency beginning when she accepted a position as Special Agent in Charge of Rota, Spain after Agent Anthony DiNozzo who Shepard had originally offered the position to declined the promotion, DiNozzo instead preferring to remain a part of the Major Case Response Team headed by Leroy Jethro Gibbs while also retaining his position of second-in-command and Senior Field Agent of the team.
As such, EJ took the position and departed for Rota, Spain, staying for an undisclosed amount of time and presumably during that time, she met her future colleagues, Agents Gayne Levin and Simon Cade.
NCIS Season 8
EJ made her debut appearance in the Season 8 episode, One Last Score (episode) where she arrived at the main NCIS Headquarters at Washington D.C., having been transferred there due to budget cuts. She then met her NCIS colleagues for the first time and even developed an attraction to DiNozzo.
In Two-Faced (episode), NCIS Director Leon Vance revealed to Gibbs that EJ had been transferred to D.C. to investigate murders committed by a lone figure called the "Port-to-Port Killer" who had a method of killing Naval personnel who made port in various cities across the world. As such, EJ became the lead Agent on the chase, drawing Gibbs's anger as he objected to both her methods and her perceived inexperience.
At some point during Dead Reflection (episode), EJ and her team took over the case. The team then headed for Hawaii during Baltimore (episode) to investigate a new lead before returning to D.C. in Swan Song (episode) where they discovered that the true identity of the killer was Lieutenant Jonas Cobb.
EJ and her team headed to an unknown area to find a witness to Cobb's murders, only to discover too late that Cobb himself had arranged the trap.
Upon arriving in the warehouse, EJ, Cade and Levin were all ambushed. Cade and Levin were shot and EJ was later kidnapped despite putting up a brave fight against Cobb.
A while later, Gibbs and DiNozzo arrived on the scene and discovered that Cade was barely still alive and that Levin had already bled to death. Leaving DiNozzo to look after Cade, Gibbs went in pursuit of Cobb and soon discovered EJ who had been locked in the trunk of her car with plastic wrapped around her head.
Following their return to NCIS headquarters, EJ was left guilt-ridden and struggling to cope with what had happened. She then secretly removed a microchip from Levin's wrist, not knowing that the Autopsy camera was recording it.
Agent Ziva David later disappeared, the team having suspected that Cobb had captured and while in a room, met Cobb who instantly surrendered to them.
Unfortunately, it was all a rue as while Cobb was being escorted to lockup, he knocked one NCIS agent unconscious, took Medical Examiner Assistant James Palmer captive and forced EJ to drive him out of NCIS using the autopsy van with the threat of killing Palmer.
It was revealed that Cobb's targeting of EJ was solely because of her relationship to SecNav, who he blamed for what he had become.
Gibbs, Vance, and Davenport pursued him back to the office building where he ambushed EJ's team before, and found him "training" three hooded figures, who were bound and being sprayed by freezing water.
It was also revealed that this building was where Cobb had been originally trained, and that the three people he were torturing were Trent Kort, whom he apparently captured earlier; Palmer; and EJ. When confronted, he attacked Davenport, but Gibbs and Vance who had been covering Davenport responded by shooting Cobb to death with the intensity of the bullets sending Cobb crashing out of the window and landing on a car.
Following the death of Cobb and the loss of both of the Agents under her command, EJ took a sabbatical to consider where she wanted to go with her life. She and DiNozzo apparently continued their relationship, while she finally gained Gibbs's respect, who offered a her a welcome back at any time.
NCIS Season 9
Several months later, though, in the Season 9 premiere episode, Nature of the Beast (episode), EJ was seen attempting to return the microchip, which apparently was capable of unlocking any electronically secured door, to Navy Captain Felix Wright, a member of a Special Operations team with the call-sign "Phantom Eight". This eventually ensnared her in the machinations of the team's commander, a Naval officer who was stealing the microchips (each of which are able to unlock all electronic doors) from deceased members of his team and attempting to sell them to international bidders. DiNozzo had also been secretly assigned to recover the microchips and eliminate the mole who had been stealing the chips, under orders from newly appointed SecNav Clayton Jarvis and without the knowledge of anyone else including Gibbs, McGee and Ziva themselves.
Ultimately, Captain Wright was killed by Jonathan Cole, a member of the Phantom Eight, who was also posing as a fake FBI agent named Stratton. Wright's microchip was removed by the imposter who then began targeting EJ in an attempt to steal the microchip she was in possession of. She made contact with Tony at this time, and the two came under fire from Stratton. They fled to Washington, where they asked for Gibbs's help.
Special Agent Cade, the only other surviving member of EJ's original team from Rota, was brought into Washington soon after. Unknown to EJ, SecNav Jarvis believed him to be the mole, and had ordered Tony to kill Cade. EJ, Tony, and Cade were all lured into a trap, after Cade returned to Washington, receiving text messages appearing to be from each other to meet in an alley at night. Tony attempted to arrest Cade, but the three discovered that neither EJ nor Tony had sent the texts and that second, the imposter struck, shooting all three. Cade was shot through the heart, killing him instantly; EJ was shot in her ribs, below her left breast; and Tony was shot in the shoulder, though his Kevlar stopped the bullet. Tony hit his head as he fell and received a concussion, resulting in him suffering temporary memory loss while EJ managed to escape before the shooter, revealed to be Cole, could kill her.
EJ, after escaping the ambush, went into hiding, living off the grid. She decided not to contact any friends or family, as she believed Tony was killed in the ambush and blamed herself for the deaths of both agents under her command, and her boyfriend. Even after discovering that Tony was alive, she chose not to contact him, fearing that she would place him in danger. Tony, meanwhile, searched for her for some time, but ultimately gave up, believing her to be dead.
After approximately a year on the run, the assassin who was attempting to kill her, Jonathan Cole (whose alias was Casey Stratton), tracked her down in New Jersey. She fled, calling an old friend from Spain to aid her, but an assassin hired by Cole, Dane Marseilles, caught up with them. He failed to kill EJ, but succeeded in killing the Naval officer she had hired. She again fled the scene, but Gibbs and his team caught up with her, although Tony was shown to be very hostile towards EJ for abandoning him in the alleyway.
After learning of her predicament, Tony and EJ were sent to a safehouse in the mountains while Gibbs, McGee, and Ziva attempted to hunt down Cole.
In the safehouse, EJ and Tony came to terms with their confused feelings for each other. EJ encouraged Tony to seek out a relationship with Ziva, whom she believed was right for Tony. Soon after, they learnt that Cole had finally been captured. The two agents came out of hiding and, though they expressed interest in resuming their former romantic relationship, EJ ultimately decided to leave as friends with Tony, and the two parted ways for good. EJ returned to her family home, where her mother was staying, for a long vacation and rest.
Relationships with other agents
EJ's relationship with Tony was a romantic, although frequently interrupted, relationship. Tony expressed interest in EJ from her first appearance, especially after learning that she had received the position in Rota, Spain, that he had turned down. Eventually, Tony goes down to intercept her as she showers before heading home, where they both succumb to their attraction for each other. They date for the better part of a year and a half, a longer relationship than any other that DiNozzo had been in. Both maintained their relationship, even after Gibbs warned both of them to end it, showing genuine love for each other - Tony even confided to Ziva, at the end of the Season 8 episode Two-Faced (episode) that "...I understand this one [EJ]. I understand her. That's why it's working." Eventually, however, EJ disappears when an assassin attempts to kill her, leaving Tony in the ambush where she believed him to be dead. When she reappears in the Season 9 episode Housekeeping (episode), Tony confronts her about leaving him and disappearing, saying that "You have a habit of disappearing a lot...consequently, I spend a lot of time looking for you. A lot of time."
While in a safe-house, the two agents reconcile with each other. However, though they both still show an interest in resuming their old relationship, they decide to part friends. EJ's parting advice to Tony is to pursue a relationship with Ziva David, whom she believes to be a better match for him than her. He warns her to take care of herself again, and not to disappear from his life anymore, before she returns home.
EJ's relationship with Gibbs was extremely tense during her time in Washington D.C. Gibbs disapproved of her methods and about the fact that she was placed in charge of the Port-to-Port killer case, over him. He also disapproved of her relationship with DiNozzo due to his 12th Rule and the fact that she was related to SecNav. However, over the course of the case, she eventually earned Gibbs's respect, to the point at which when she left for a vacation, he offered her a welcome back to NCIS at any time.
Ziva and EJ had a tenuous relationship at best when they first met. Ziva believed that EJ was an annoying and scatterbrained agent. However, over time, they became friends, particularly after Tony and EJ began dating. Later, they discussed their plans outside of NCIS for their futures, and EJ ultimately encouraged Tony to act on his feelings for Ziva, believing her to be a better match for Tony, even though Tony disagreed originally, citing Gibbs's 12th Rule restriction on dating NCIS co-workers.
EJ was close friends with Levin, who was an agent under her command. She was shown to be protective of him, and considered him a highly talented agent. She also felt a deep loyalty towards him, enough that she risked her life for a year and a half to fulfill his last request of her. Levin's death had a profound effect on EJ, who blamed herself for his death and suffered from survivor's guilt. Eventually, with Tony's help and the killing of Levin's murderer, EJ was able to recover, though she still blamed herself a year and a half later for his death; it was partially due to Levin's death that she separated herself from all her friends and family, and her lover, Tony, fearing that she would get another person she was close to killed.
EJ was also close friends with Cade, another agent under her command. She considered him, too, to be a very talented agent, and, like with Levin, felt guilt over his injuries and eventual death. Cade's death, coupled with the death of Agent Levin, left EJ guilt-wracked, blaming herself for their deaths. As a result of the guilt that she felt over their deaths, when EJ again became a target for unknown agents, she chose to separate herself from her friends, family, and lover, fearing that she would get another person she was close to killed.
Based on how she acts in Two-Faced (episode), EJ appears to a be a highly career-orientated agent just like Jennifer Shepard the director who promoted her , focused on advancement and securing favor with superior officers. Outside of her professional life, she is mostly shown as a light-hearted person, teasing and flirting with Tony constantly; however, she is dating Tony and is only shown outside of her professional life with him.
She is friendly with her subordinate team from Rota. She is willing to flirt with suspects in order to trick them into divulging secrets, a technique that Gibbs wholeheartedly disapproves of. It appears that she believes in unconventional methods of persuasion and investigation - when confronted by Gibbs after an interrogation of a suspect, he tells her the "correct" way in which to interrogate; she responds with, "Maybe that's the old-school way to do it." She appears to be one of the few agents who either do not respect, or are not intimidated by Gibbs, a trait that DiNozzo disapproves of, saying that "...if you mess with the Great White [Gibbs], you'll get bitten." When pressed about joining an agency her uncle has command of, and thereby influencing her career, she claims she earned her place in NCIS but soon stops arguing, possibly showing that she knows she was given the fast-track for being related to the Secretary of the Navy. She is also fiercely protective of those she cares about, such as Tony and her mother, choosing to go on the run and live for an entire year with no contact with them in an attempt to protect them.
Despite her natural abilities, E.J. also seems to have spectacularly bad luck. She is first stalked by a serial killer, who then attempts to kill her, then captured and tortured by the same serial killer. Following this, a Phantom Eight operator attempts to murder her four times, failing each time either due to her constant use of body armor or misinformation. She is similar to Tony in this regard, as he too is nearly killed many times over his career.