Rhaenys Targaryen (daughter of Rhaegar)

Rhaenys Targaryen, is an unseen character in Game of Thrones. She is deceased when the events of the series begin and is not expected to appear in the series.

Rhaenys Targaryen was the daughter of Prince Rhaegar Targaryen and Ellia Targaryen. She had a younger brother, Aegon. House Targaryen was deposed as the ruling house of Westeros during Robert's Rebellion. Rhaegar was killed during the fighting and Ellia and both their children were killed soon afterwards during the Sack of King's Landing.

In the books
In the Song of Ice and Fire novels, Rhaenys is the the daughter of Prince Rhaegar Targaryen and Ellia Martell and died in the Sack of King's Landing.