Countess de Stael is a lady mentioned in the Witcher novel "The Tower of the Swallow" and In the Netflix series. In the novels, she was the reason why Dandelion started to compose songs and to write poetry at the age of 19, because he was head over heels in love with the Countess. This is mentioned in "The Tower of the Swallow" by Geralt of Rivia.
In the Netflix-Series, the bard obviously had a closer relationship with the Countess and she had freshly left him again in S1/E5: Bottled Appetites, which contains many story elements of the short story "The Last Wish". In the original story, the Countess is not mentioned, but a woman named Virginia, which the bard could not conquer so far with his charms.