The Rani
The Rani is a rogue Time Lady who has known the Doctor and the Master for most of her life. A genius at bio-engineering, chemisty, physics and other science, she seeks only to dominate planets, and time, for herself. Her work lacks any ethics or morals, and she is totally obsessed with it.
Strengths:
A genuis at sceince.
A Time Lord, so she has an in-depth knowledge of time.
Weaknesses:
Becomes obsessed with her work, and can thus be manipulated, as both the Master and the Doctor showed.
Appearances: The Mark of the Rani, Time and the Rani.