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Thalia was a Time Lord and High Chancellor under President Borusa.
Like all Time Lords, Thalia was taken from her family at the age of eight for the selection process in the Drylands. Staring into the Untempered Schism as part of a Time Lord initiation rite, Thalia was inspired by what she saw in the Schism. (A Brief History of Time Lords)
Thalia became High Chancellor some time after Chancellor Borusa became President of the High Council. She, President Borusa, Cardinal Zorac, Councillor Hedin and Castellan Jerricho met to discuss the appearance of an anti-matter creature with Thalia offering her expertise on the field. After the Fifth Doctor arrived on Gallifrey and evaded the Chancellery Guard, she reminded Jerricho of how critical it was that he be captured.
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Once the Doctor and Nyssa were captured, Thalia welcomed them and, when the matter of the Doctor’s cooperation arose, she mentioned how he had failed to return Romana II to Gallifrey. She was later willing to have the Doctor executed in order to save “untold billions”, telling Hedin that they could not delay and warning Nyssa that she would die if she did not obey the President and set aside her staser.
Thalia, Zorac and Hedin were summoned to Jerricho’s office by Chancellery Guards, which offended Thalia, and were told of the Doctor’s survival. She came to believe that the President was behind it due to the use of the presidential codes, when in reality it was Hedin. After the truth was uncovered, Thalia suggested the use of a pulse loop to distract Omega, which was successful. (Arc of Infinity)
After the Death Zone was reactivated, (The Five Doctors) Thalia and Zorac were sent by Borusa to investigate. They died, potentially only moments after entering the Zone. (A Brief History of Time Lords) She was succeeded by Flavia. (The Five Doctors)
Thalia was intelligent and an expert in the field of biodata. (Arc of Infinity) She was also knowledgeable on the topics of anti-matter and travel between dimensions, prompting Cardinal Zorac to ask for her expertise on such matters during the Omega crisis. (Arc of Infinity)
Chancellor Thalia was a handsome woman in the prime of her life during the Omega crisis. She appeared middle-aged (Arc of Infinity)