The Rani (The Mark of the Rani)
The Rani is a renegade Time Lady, an evil scientific genius whose villainy comes not from the usual variety of lust for power and suchlike, but from a mindset that treats everything (including morality) as secondary to her research; she has been known to enslave entire planets in order to have a ready supply of experimental subjects and a place to carry out her experiments uninterrupted. Her major interest is in tinkering with other species' biochemistry - she was exiled from Gallifrey after some of her lab mice, as a result of an experiment, grew to enormous size and ate the President's pet cat. A past relationship between the Rani and the Doctor is hinted at, but never elaborated upon. It has been established that they are the same age.
While the Rani certainly does not share the Doctor's fondness for Earth, referring to it as a "miserable planet," it has been the focus of several of her research projects. When the test subjects on one of the planets she had enslaved became violently restless and uncontrollable, - a side affect of her experiments on them, - the Rani visited Earth at various points in its history to extract chemicals from the brains of select humans. Because the chemicals in question enabled the human brain to rest, and because the absence of these chemicals made her victims as violent and uncontrollable as those from her previous experiments, the Rani purposely chose periods of social unrest to visit, using the violence to conceal her presence and its consequences. The seige of Troy, the Dark Ages, the American War for Independence, and the Luddite Riots of the early 19th century were just a few of the historical periods she visited, before her work was brought to a halt by the arrival of the Doctor and the Master, after which the Rani became caught up in the rival Time Lords' feud ("The Mark of the Rani"). The Rani also hoped to use the Time Brain she was growing on Lakertya to correct what she considered to be errors in the universal timeline. Her first target was to be Earth, where she would prevent the extinction of the dinosaurs, creatures whose full poetntial she felt had never been truly realised ("Time and the Rani"). Prior to either of these projects, she had visited Earth in the late Jurassic and acquired several Tyrannosaur embryos.
Towards the end of the Time War, the Doctor's homeworld was destroyed and he later told his companion Rose Tyler that he was the last of the Time Lords. Whether the Rani was present on Gallifrey and was killed along with the rest is uncertain.
The Expanded Universe
The Rani once tried to trap the Doctor in his first seven incarnations in a temporal trap, hoping to add one of his companions to a menagerie of creatures with which she hoped to gain access to and control of every individual mind in the universe ("Dimensions in Time"). She was also trapped along with the Sixth Doctor and Peri in the alternate Earth of Terra Nova that had been created by the entity known as Iam. She tried and failed to manipulate the political situation existing between the three children of that reality's version of Cleopatra ("State of Change"). After being betrayed and imprisoned by her own servants, the Tetraps, the Rani achieved her escape with the help of the very persons upon whom the Tetraps instructed her to experiment ("The Rani Reaps the Whirlwind").
In her youth, the Rani was one of a group of promising young Time Lords called "the Decca" which included many future renegades, including the Doctor and the Master. Her given name at this time (or at least its first syllables) was Ushas ("Divided Loyalties").
