Second Rani

The Rani was a renegade Time Lady. She knew the Doctor and the Master when all three were young, and became an enemy of the former and an unwilling ally of the latter.

Youth and exile
The Rani (formerly known as Ushas) belonged to the Deca, the same Academy clique as the First Doctor. (PROSE: Divided Loyalties)

Ushas was the same age as the Doctor. (TV: Time and the Rani) The Doctor was once invited to Ushas' 94th birthday party. (PROSE: The Death of Art)

At the Doctor's graduation, there was an incident involving Ushas and a giant rat. (COMIC: Party Animals)

Ushas was exiled from Gallifrey after some of her lab mice, as a result of an experiment, grew to enormous size and ate the Lord President's pet cat. They also bit the President himself, triggering a Regeneration. (TV: The Mark of the Rani) However, she hijacked the TARDIS delivering her to her planet of exile and became a renegade known as the Rani. (PROSE: Divided Loyalties)

Presence in Earth history
While the Rani certainly did not share the Doctor's fondness for Earth (she referred to it as a "miserable planet"), it was the focus of several of her research projects.

At some point, she had visited Earth in the late Cretaceous and acquired several Tyrannosaur embryos.

When the test subjects on Miasimia Goria, a planet she had enslaved, became violently restless and uncontrollable as a side effect of her experiments on them, the Rani visited Earth at various points in its history to extract chemicals from the brains of select human specimens. Because the chemicals in question enabled the human brain to sleep, and because the absence of these chemicals made her victims as violent and uncontrollable as those from her previous experiments, the Rani deliberately chose periods of social unrest to visit, using the violence to conceal her presence and its consequences. She visited the Trojan War, the Dark Ages, the American War of Independence, and finally the Luddite riots in the village of Killingworth during the early 19th century where she used the local bath house as her base, posing as the old woman in charge of the premises.

and, shortly after, the Sixth Doctor, interrupted her work. The Doctor sabotaged the navigational system of the Rani's TARDIS, trapping the Master and the Rani inside as time spillage caused the Tyrannosaur embryos to grow at a dangerous rate. (TV: The Mark of the Rani)

On Terra Nova
The dinosaur grew to such a size that it broke its neck on the ceiling, but the Rani had been left adrift in her TARDIS when the Master escaped from her by detaching the console room from the rest of her TARDIS. Shortly afterwards, from the Doctor's subjective point of view, the Rani was also trapped along with the Sixth Doctor on Terra Nova, which the entity known as Iam had created. She had in the meantime tried and failed to manipulate the political situation existing between the three children of that reality's version of Cleopatra. However, she did manage to escape Terra Nova, by using the entity to create a new TARDIS console room to replace the one the Master escaped in. (PROSE: State of Change)

On Koturia
On the planet of Koturia, the Rani went by the name of Lania. She was genetically modifying pterodactyls in order to take blood and tissue samples of Koturians for the purpose of learning about Phasing. She believed that she could learn of a way to control the outcome of a regeneration by learning how Koturians control their appearance when they Phase. She immersed herself in Koturian culture and became engaged to Jonos, an upper class Koturian. However, their marriage ceremony was crashed by the Sixth Doctor and Peri Brown, the former of whom was invited as an old friend of Jonos's father, Evris Makshi. Although they were too late to stop the physical ceremony, it failed to result in Jonos completing his Phase because both bride and groom have to be in love with one another for for the Imori stone to work and the Rani was only pretending to be in love with Jonos. Although the Rani claimed to be immune to emotion, the Doctor believed she felt some kind of emotional attachment to her work, as she did beg him to not destroy her research by saying "please." (PROSE: Something Borrowed)

On Tetrapyriarbus and Lakertya
On the planet Tetrapyriarbus, the Rani made the acquaintance of, and decided to employ, the Tetraps, led by Urak. With them, she invaded the peaceful planet Lakertya and put into motion a complex plan. The Rani abducted eleven scientific geniuses from across time and space, including Albert Einstein of Earth. Finally she decided to "collect" the Doctor and attacked his TARDIS, causing the ship to go through turbulence. The Doctor was knocked unconscious as a result, triggering the regeneration into his seventh incarnation.

The Rani channelled the intellects of the geniuses into a giant artificial brain which she believed could find the secret to manipulating strange matter, the key to making the planet of Lakertya into a Time Manipulator in order 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 potential she felt had never been truly realised. She considered the death of the native Lakertyans a small price to pay.

She used the artificial brain to find the answer as to how to create a lightweight substitute for strange matter. When it was devised she sent a missile containing the substance aimed at a strange matter asteroid. However, the Doctor destroyed the brain and redirected the missile. Urak betrayed her, leading the Tertraps against her, and they placed her under house arrest in her TARDIS on Tetrapyriarbus. The Rani was 953 years old when these events took place. (TV: Time and the Rani)

After these events occurred, the Tetraps faced a food shortage crisis, while Urak managed to have the Rani put on trial, with the death sentence. She would have to solve the food shortage, otherwise her sentence would commence. Two human and two alien prisoners were to be test subjects for the Rani's experiments in an attempt at solving the crisis. The Rani, however, teamed up with the four "guinea pigs" and managed to escape the planet. Each then went their separate ways, with the Rani swearing to teach Urak a lesson and retrieve her TARDIS from him. (AUDIO: The Rani Reaps the Whirlwind)

Ultimate fate
Father Kreiner killed the Rani and the clone of the Master and kept their heads as trophies during the events of the Second War in Heaven. (PROSE: Interference - Book One) However, many aspects of the War took place in another timeline due to the Eighth Doctor's intervention, therefore leaving the ultimate fate of the Rani unknown. (PROSE: The Ancestor Cell)

Alternative timeline
In a alternative timeline, the Rani cooperated alongside the Master, the Monk and Drax to try to destroy the world using a DNA recombinator, turning the human race into a gestalt consciousness which could be used as a weapon to conquer the universe. (PROSE: The Quantum Archangel)

Personality
The Rani was an evil (or, arguably, simply amoral) scientific genius whose villainy came not from the usual variety of lust for power and suchlike, but from a mindset that treated everything (including morality) as secondary to her research. She was known to enslave entire worlds in order to have a ready supply of experimental subjects and a place to carry out her experiments uninterrupted. Her major interest was in altering the biochemistry of other species.

While she did appear evil, she found the Master to be truly evil and therefore stupid. She simply did evil things because she felt it was necessary to her work. When the Sixth Doctor tried to convince her not to experiment on humans, she called them carnivores and asked if they ever thought of the lesser species when they sunk their teeth into pork chops. This shows that the Rani may have had a conscience of some kind, also shown when she was willing to destroy her test subjects because they would've killed the Doctor. (TV: The Mark of the Rani)

Behind the scenes

 * Kate O'Mara has, to date, portrayed the Rani in all of her television appearances – as well as her single (to date) audio appearance.
 * In August 2012 Steven Moffat stated that "he had no reason to bring back the Rani", thus putting an end to the rumours of her return to the series.
 * Russell T Davies has said that if he had brought back the Rani, he would have cast actress Ruthie Henshall in the role.