She served as an ally and hindrance to both Gibbs and Fornell up until her death in January 2015 at the hands of terrorist Sergei Mishnev.
Diane was Gibbs's first ex-wife (second wife) after Shannon Gibbs's death who, after the divorce, took his money, including his grandfather's watch.
Diane was aware of Shannon and Kelly, as seen with Diane's statement in Devil's Triangle (episode) about her being "human anti-depressant" for Gibbs, and her comments about how he was "her Shannon". Also, in this episode, she returns his Grandfather's watch, stating that she still had a mutual relationship with Gibbs.
She was also married to a man named Victor Sterling, who is a Homeland Security officer. She invited both Gibbs and Fornell to the wedding, as seen in the episode Tell-All (episode), but neither of them accepted. After their wedding, Gibbs sent them a waffle iron. Prior to her marriages, Diane worked with the IRS as an auditor. In the episode Devil's Triad (episode), it was stated that she wished to separate/divorce Victor.
After her marriage to Victor she returned to work and eventually was promoted to the rank of investigator in the enforcement division.
Afterwards, she attempted to investigate embezzlement within the IRS. Gibbs and Fornell, not knowing about her promotion, initially thought she lied when questioning her during her investigation and attempted to arrest her, as they were currently investigating a case where a discharged Navy officer attempted to kill Fornell, but he shot the ex-officer in self-defense. After this revelation, she was later forced by Director Vance, alongside Fornell and Gibbs, to work together. She was also having marital problems with Victor Sterling, believing that he was planning on leaving her.
This eventually resulted in Special Agent McGee sleeping next to her as a means to comfort her, causing some anger from Fornell. They eventually found out that a rogue IRS check handler was responsible for both the attempted hit on Fornell (although the target was actually intended to be Sterling, as she still used Fornell's last name in legal documents) and embezzling IRS funding to smuggle items to other countries inside fish.
In Devil's Triad (episode), it was revealed that she and her ex-husband, Tobias Fornell were planning on getting together but their teenage daughter, Emily Fornell was not pleased with the development.
NCIS Season 12
In Check (episode), Diane assisted Gibbs's third wife Rebecca Chase in her efforts to fulfill the requirements of her sobriety program.
However, after she ended up discovering Rebecca and Gibbs sleeping next to each other on the couch with McGee, she told them off (ironically despite she and McGee being caught in a similar situation earlier by Gibbs and Fornell).
Later, Diane and Gibbs arrived at a car park, both having been unknowingly lured there thanks to Sergei who had faked a call from her to Gibbs, the topic being classified intelligence.
Upon realizing that this "meeting" was a trap, Gibbs attempted to get Diane to safety, only for Sergei to strike which the terrorist did by shooting Diane in the head via a sniper rifle.
As Diane collapsed to the ground, dead and with a bullet hole in the center of her forehead, Gibbs was left soaked with Diane's blood, the blood on the left side of his neck and nearly covering his left ear too with Diane's death matching that of NCIS Special Agent Caitlin Todd whose murder had occurred ten years ago during Twilight (episode).
In the aftermath of Diane's death, Gibbs ordered that protection be assigned to his two surviving ex-wives as well as Diane's teenage daughter, Emily Fornell as Gibbs feared that they could become potential targets.
Fornell himself, however, was being debriefed at the FBI headquarters and thus could not be notified immediately at the time of Diane's murder, although Gibbs eventually told him upon being called after he failed to avenge her in a raid at Mishnev's apartment.
Following her death, Fornell began self-destructing, even getting busted by the police for drink-driving while Emily herself struggled to cope with her emotions and keeping her father on track.
In Cabin Fever (episode), Fornell shot and killed Sergei, ultimately avenging Diane.
- Her computer password was "Tobiascheap".
- Diane calls Gibbs by his first name, Leroy, as does Victor.