Trystane is the son of Doran Martell (Alexander Siddig), the Prince of Dorne and Lord of Sunspear. In the A Song of Ice and Fire books, Trystane is the youngest of Doran’s three children, but since he is described as the heir to Dorne in the press release announcing his casting, it’s safe to say that his two older siblings from the books did not make it into the show. So what does this mean?
It probably means Trystane is going to be a much bigger player on the show than in the books. The show has also aged up the character from about 13 in the books to 18 on the show. He is also betrothed to Myrcella Baratheon (Nell Tiger Free), Cersei’s daughter, who is also being aged up for Season 5. Making these two characters young adults suggests the possibility of a romantic storyline that does not exist in the books.
To be honest, Trystane doesn’t do much of anything in the books other than become betrothed to Myrcella and play a couple board games with her, so we’re anxious to see how the show utilizes him in Season 5.