Dexter (42)

Miss Dexter was a ruthless security agent working for Mr Saxon, entrusted with the task of finding the Tenth Doctor and his companions. Miss Dexter enlisted Francine Jones's help, tapping and tracing Francine's phone conversations with her daughter Martha, who was travelling with the Doctor at this time. (TV: 42)

She was at the Jones' house when Mr Saxon ordered her to arrest all of Martha Jones' family and bring them to the Valiant. After a brief struggle, Dexter managed to arrest Francine, Clive, and Tish Jones, during which she ordered her guards to open fire on Martha.(TV: The Sound of Drums)

Following the Toclafane invasion she continued working for the Master; following his orders, she put UCF Griffin in charge of finding Martha. (PROSE: The Story of Martha)

Behind the scenes
This character's name was never revealed on screen. Like Trinity Wells, her name is derived solely from the credits. In her case, she was the "Sinister Woman" in 42, but "Miss Dexter" in the cast of characters for The Sound of Drums.