A police detective in Baltimore, Maryland, Diana was partnered with Peter Sheridan when they arrested Dean and Sam for two murders. She tried to convince Sam to testify against Dean, but Sam refused. Although she initially did not believe the brothers when they talked to her about supernatural occurrences, she changed her mind upon encountering a ghost in the station's bathroom.
After Sam escaped, she got Dean to tell her where he was, and met up with him. The two found the body of Claire - the ghost believed to be responsible for the murders. Sam, however, deduced that Claire was not behind the killings - rather she was trying to warn them about the real killer. Diana realized that her partner and boyfriend Peter Sheridan was behind all this, and she and Sam tracked him down just as he was about to execute Dean.
Pete tried to explain his actions and told Diana they could blame everything on the Winchesters and be together. Diana however shot him, non fatally though. Sheridan got back up and attacked her. Suddenly the ghost of Claire appeared, distracting him, and Diana shot Pete again, this time killing him.
Afterwards, she let Sam and Dean go, as she now knew the truth about the supernatural entities they were hunting.
Months later, public defender Mara Daniels contacts Ballard after Sam and Dean are arrested in Arkansas. Daniels reports to Special Agent Victor Henriksen that Ballard swears up and down that the Winchesters saved her life and helped her catch a killer. Ballard's statement along with other inconsistencies in the case against the Winchesters causes Daniels to start to doubt the Winchesters' guilt.
It is revealed by New York Police Detective Marina McBain tells the Winchesters that Ballard has been recruited into a small country-wide group of cops who know of the supernatural and help hunters. She also tells them that Ballard is under investigation for her role in the events of The Usual Suspects and while she will probably be cleared, she likely won't be allowed to work Homicide anymore.
- Season 2
- Supernatural: Nevermore (mentioned only)
- Diana is one of the five female Winchester allies to have not died and yet not made an appearance since their debut, pre-Season 4. The others are Cassie Robinson, Kathleen Hudak, Mara Daniels and Tamara.