She is a Homeland Security Agent that works in Washington D.C. along with her partner, Eric Cortez. After a demon killed her partner, she began working with Ichabod Crane to hunt down the creature that killed her partner and many other civillians. She is the mother of Molly/Lara Thomas, who was revealed to be the new Witnesses as well as the Oracle.
The Missing Episodes of Sleepy Hollow
Diana was determined to reverse Ichabod's curse after he made a deal with the Devil. She didn't do much since she was a skeptic, though she managed to hold her own as the group lived through the Tribulations and battled the Horsemen, excepting Death. It was discovered that she wielded the virtue of Justice, and it was passed down by Abbie herself, as well as her mentor, August..
She later gathered some supplies with Lara, then Ichabod was sent to Hell after losing against the Horsemen of War and Death. She went with the team and she was later baptized by Jenny at Limbo. She continued to trek with them, learning of Hell's existence, and soon was persuaded by Mitch at Lust. Thanks to the team's support, and the defeat of Cleopatra by Gavin and Jenny, she was spared. She later came across Ichabod. After fighting Katrina and Benedict, as well as freeing their souls, they saved him as Lucifer was escaping. She then survived the plagues, the apocalypse, the Horsemen, and Lucifer. She then released her virtue restoring the world.
Weeks later, she was at the ceremony as Gavin and Jenny were bestowed as Future Witnesses.