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Lydia van Bredevoort was Vilgefortz's assistant and personal secretary, a sorceress, and a painter. She was a woman with a very strange face and very nice, expressive eyes. The lower part of her face was actually an illusion. Four years prior the events in Blood of Elves, Lydia, at Vilgefortz's insistence, had taken part in some experiments conducted on a mysterious artefact discovered in a necropolis. It turned out to be cursed and although it "activated" only the one time, three of the five mages who participated where killed on the spot. A fourth lost his eyes, his hands and went mad. Lydia escaped with burns, a mangled jaw, and throat and larynx mutations that stubbornly resisted all attempts at regeneration. She was left with telepathy as her only form of communication.

'And that girl with a strange face walking behind Vilgefortz?' 'That's his assistant, Lydia van Bredevoort,' said Yennefer coldly. 'A meaningless individual, but looking her directly in the face is considered a serious faux pas.'
— pg(s). 117, Time of Contempt (UK edition)

She was also hopelessly in love with Vilgefortz but too proud to admit it and he in turn was too principled to take her as his lover because he did not actually love her, though it was suggested. This situation, strange though it was, seemed to suit them both.

She died, likely by her own hand and at Vilgefortz's order, during the coup on Thanedd Island. When she died, the illusion that had hidden the monstrous result of the failed experiment dissipated and her true face was revealed, much to the horror of the witnesses.

Geralt recognised the body as that of Lydia van Bredevoort. He knew her by her hair and silk dress. He couldn't have recognised her by her face because it was no longer a face. It was a horrifying, macabre skull, with shining teeth exposed halfway up the cheeks, and a distorted, sunken jaw, the bones badly knitted together. [...] The lower part of Lydia van Bredevoort's face, as seen in society and her portraits, was actually an illusion. Experiments on a mysterious artefact had left her with burns, and throat and larynx mutations.
— pg(s). 164, Time of Contempt (UK edition)