Shiera Seastar

Shiera Seastar is an unseen character in Game of Thrones. Sge is not expected to appear in the series, being long dead by the time it begins.

Background
Shiera was a bastard daughter of King Aegon IV Targaryen, also known as "Aegon the Unworthy".

Season 1
Shiera, along with her siblings is mentioned in the House Targaryen entry of the book The Lineages and Histories of the Great Houses of the Seven Kingdoms.

In the books
In the A Song of Ice and Fire novels, while King Aegon IV had many bastard children, the mothers of four of them were also noblewomen, which encouraged him to formally acknowledge them. The four Great Bastards were Daemon Blackfyre (whose mother, secretly, was Daena Targaryen), Aegor "Bittersteel" Rivers (whose mother was a member of House Bracken), Brynden "Bloodraven" Rivers (whose mother was a member of House Blackwood, and Shiera Seastar (whose mother was Serenei of Lys, from an ancient Valyrian noble family).

On his deathbed, Aegon IV foolishly had them all legitimized, which led to strife between them and his legitimate heir King Daeron II, tensions which ultimately split realm apart in the devastating civil war known as the Blackfyre Rebellion. Bloodraven sided with their half-brother King Daeron II Targaryen, while his other half-brother, Bittersteel sided with his half-brother Daemon Blackfyre.

Bittersteel and Bloodraven hated each other, because they both competed for the love of their half-sister Shiera Seastar. Bloodraven asked her to marry him dozens of times and she refused, but she did let him into her bed - though she had many lovers. Shiera, however, completely rejected Bittersteel, and for this he never forgave Bloodraven. It also didn't help that Bittersteel's mother was a Bracken and Bloodraven's mother was a Blackwood, as those two noble families had been feuding for thousands of years. Bittersteel became a great champion and general for the Blackfyres, while Bloodraven became a great champion and general for the Targaryens.