He was one of Yennefer's long-time lovers. He challenged Geralt to a duel for the right to exclusive access to Yennefer's affections.
Istredd proposed to Yennefer while she was with Geralt in Aedd Gynvael. He promised her stability, a home, a future, and the child(ren) she's longed for. However, Yennefer left the city even before Geralt, and Istredd ended up helping Geralt when he faced Dermot Marranga and his men.
Played by Bronisław Wrocławski.
In the Netflix series
In the show he is the first one who is aware of Yennefer's powers when the then disfigured girl suddenly appears before him through a portal. Their love story is described a bit more elaborate than in the short stories. He misuses her trust when he reveals her elvish ancestry to Stregobor, who also has a much larger role than in the books.