Diane is 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 is currently 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.
- Diane is portrayed by actress Melinda McGraw, best known for her role as "Barbara Gordon" in The Dark Knight.
- Her computer password is "Tobiascheap".
- Diane calls Gibbs by his first name, Leroy, as does Victor.