Rani
The Rani was a Renegade Time Lady, an amoral scientist who will use any means to achieve her ends. The Rani was a contemporary of the Doctor. She was exiled from Gallifrey when genetically altered mice that she created devoured the President of Gallifrey's cat, before attacking the President himself.
The Rani was first encountered as a renegade by the Sixth Doctor in England just prior to the Industrial Revolution, where she posed as a healer. However, she was actually extracting brain fluids from local miners that they needed for sleep, causing them to become dangerously aggressive. Her experiments were ruined by the Doctor and the Master, the latter of which stole her collected brain fluid. The Doctor defeated them both by tampering with the Rani's TARDIS, trapping both the Master and the Rani inside, and sending them to the edge of the universe. They were also forced to deal with a rapidly-maturing Tyrannosaur accidentally released from the Rani's storage.
After escaping the dinosaur and the Master, the Rani gathered members of the Tetrap race as servitors, then proceeded to the planet Lakertya. Setting up a base there and subjugating the local population, the Rani began capturing great geniuses from across time and combining their intellects to create a giant brain. With that brain, the Rani planned to revert the universe to a primordial state, examining it and reshaping it as she wished. However, her plans were disrupted after she crashlanded the newly-regenerated and highly confused Seventh Doctor on Lakertya, and tried to enlist his help. Despite her attempts to deceive him, the Doctor rallied the Lakertyans against her and destroyed the giant brain. The Rani fled in her TARDIS, but she was captured by the Tetraps, who decided to put her to their own uses.
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In the non-canon for-charity special Dimensions in Time, the Rani attempted to capture the first seven Doctors and a number of their companions within a time anomaly centered around a small section of London (the region of the series Eastenders, specifically). However, she was defeated.
