Domeric Bolton

Domeric Bolton is an unseen character in Game of Thrones. He was the heir of House Bolton, but is now dead.

Biography
Domeric Bolton was the only trueborn son of Lord Roose Bolton, the Lord of the Dreadfort. Following his death, Lord Bolton's only living offspring is Ramsay Snow, his bastard son.

Behind the scenes
So far, Domeric has only been indirectly mentioned by GRRM in the "Bastards of Westeros" promotional video.

In the books
In the A Song of Ice and Fire novels, Domeric Bolton was the only son and heir of Lord Roose Bolton and Lady Bethany Ryswell, his second wife. According to Roose, Domeric was quiet but accomplished; he read history, played the harp and rode as if born on a saddle. Lady Barbrey Dustin, his aunt, was quite fond of Domeric, who had served as a page at Barrowton.

Roose suspects, and is probably correct, that Ramsay murdered Domeric in order to be Roose's only heir. It is not surprising that Ramsay is indifferent to the taboo of kinslaying.