Darkel

Inquisitor Prime Darkelatraquistahastrad (Darkel for short) of the House of Jurisprudence served on the High Council of Gallifrey.

Involvement in the Doctor's trial
Darkel presided over the Sixth Doctor's trial on the Time Lord Space Station Zenobia. The Valeyard acted as prosecutor. (TV: The Mysterious Planet, TV: Mindwarp) The Doctor acted as his own defence lawyer. (TV: Terror of the Vervoids)

An honest woman, Darkel presided over a show trial meant to draw attention away from the corrupt High Council. With political unrest demanding a new Lord President and the Doctor unwilling to take the job himself, he gave a symbolic vote of confidence to Darkel before departing in his TARDIS. (TV: The Ultimate Foe)

The Valeyard on Trial
Darkel oversaw the trial of the Valeyard. She brought the charges against him whilst the Sixth Doctor provided his defence. After the Valeyard was seemingly executed, Darkel followed the Doctor to a satellite of Etarho, where she discovered that the Valeyard had survived. She witnessed the Valeyard escape from her grasp and escaped the planet in the Doctor's TARDIS. The Doctor warned Darkel that the Valeyard would try to assassinate her now that he was free. (AUDIO: Trial of the Valeyard)

Later events
By the time of Romana's presidency, Darkel had become a deeply conservative and ferociously ambitious figure in Gallifreyan politics. She vehemently opposed Romana's reforms, particularly her invitation to off-worlders to become students of the Time Lord Academy. She began her campaign innocuously when she was invited by Coordinator Narvin, another opponent of Romana, to undertake an inquiry into Romana's actions. (AUDIO: The Inquiry) After a mutually satisfactory resolution to the inquiry, Darkel conspired with Narvin and also with the disgraced Andred to overthrow Romana by any means. She allowed Andred to escape and had Narvin frame him for the murder of the Archivist. (AUDIO: Lies) During this period, Darkel received advice and guidance from a disembodied voice in the Vaults. Unbeknownst to her, it was the voice of Pandora.

She continued to plot against Romana with anyone whom she thought to be useful. When she learned that Romana's genetics had been shaped by Pandora, she tried to convince Cardinal Braxiatel it was too great a risk for her to be president. Darkel manoeuvred Braxiatel into exile when he absorbed aspects of Pandora into his mind. With him out of the way, she challenged Romana for the presidency. She also plotted with Commentator Antimon to plant bombs aimed at the alien students. Antimon, however, was a follower of the Free Time movement, using Darkel's patronage to his own ends. He tried to kill Romana, Darkel, and the High Council when they met for the election. The plot foiled, Romana declared herself Imperiatrix, effectively dictator, denying Darkel the election. At that moment Pandora took physical form and also claimed the Imperiatrix title. Darkel realised too late that she had been duped by Pandora. (AUDIO: Imperiatrix)

Desperate for survival, Darkel sided with Pandora and prosecuted the civil war against Romana on her behalf. She proved to be a ruthless and determined war lord, but her arrogance led to her capture by the enemy and being on the losing side. (AUDIO: Warfare) She was pardoned. With Romana incapacitated, she was appointed by Acting President Valyes as his successor. The disputed presidency again had to be put to a vote, but again Darkel was thwarted, this time by the returning Braxiatel. Provoking him into releasing the Pandora entity, Darkel discovered that Pandora had been targeting her all along, and that Brax had been protecting her. Darkel's subsequent possession by Pandora destroyed her mind and killed her. (AUDIO: Mindbomb)

Behind the scenes

 * On-screen Darkel had no name given to her, other than "the Inquisitor". The name Darkel was given to her in the Big Finish Productions Doctor Who Audio spin-off series Gallifrey.