She was played by Megan Gallagher.
Dianne Hewlett was the Chief of Staff for Senator John Wilton, but she was on her way to retiring from her position. She was revealed to be part of a conspiracy to set up former senator Dean Kelman, which included a fabricated story that he was having an affair with his aide, Julia Helmsworth, who later went missing. The scandal cost Kelman his marriage and his job, and he ended up committing suicide. Dianne was racked with guilt for her participation in the conspiracy, but she took a twisted method in an attempt to make amends, doing so by killing everyone who was responsible. She acquired a sword hilt that belonged to Alfred Dreyfus; an artifact that had the power to kill those who lie after looking at it. Dianne used the artifact on Wilton as well as a reporter, and later used it on Ted, a Secret Service agent and a friend of Pete and Myka's, when they began getting too close to the truth, which included that Dianne had been sending checks to Julia.
Dianne was later shown smirking evilly in front of Julia's house, as she held up the sword and put Julia under its control. After entering the home, the evil Dianne tricked Julia into lying, and she begins to cough up water and drown, just like Dianne's past victims. She later puts the spell on herself and lies in an attempt to commit suicide, but Pete and Myka catch her and gained possession of the artifact. Not only was Julia saved, but Dianne was prevented from taking the easy way out, forcing her to answer for her actions.