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, while at a car park, Diane and Gibbs spoke for a time with Gibbs attempting to get Diane. Unfortunately, Sergei Mishnev who had been overseeing the meeting shot Diane in the head, killing her instantly while Gibbs was left drenched with Diane's blood with the circumstances of Diane's murder matching those of NCIS Special Agent Caitlin Todd whose murder had occurred ten years ago. In order to ensure Gibbs was nearby to witness her death, Mishnev also sent a faked call from her to Gibbs requesting that she and Gibbs meet at the location regarding classified intel.
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. 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. In addition, James Palmer, the assistant mortician to Donald Mallard, refused to aid in the autopsy, as he had "hit his limit."
In Cabin Fever (episode), Diane was avenged when Fornell shot and killed Sergei.
- Diane was portrayed by actress Melinda McGraw, best known for her role as "Barbara Gordon" in The Dark Knight.
- Her computer password was "Tobiascheap".
- Diane calls Gibbs by his first name, Leroy, as does Victor.