Date of birth
I'm brokering an alliance with House Martell of Dorne. Princess Myrcella will wed their youngest son when she comes of age." ―Tyrion Lannister
Trystane Martell is an unseen character in the second season. He is first mentioned in " What is Dead May Never Die".
Trystane is a member of
House Martell, the ruling house of Dorne.
Trystane is proposed as a potential marriage candidate for Princess
Myrcella Baratheon by her uncle, Hand of the King Tyrion Lannister. The alliance with the Prince of Dorne is accepted and Myrcella is sent to Dorne as a ward of House Martell. 
Season two appearances
The North Remembers
The Night Lands
What is Dead May Never Die Garden of Bones
The Ghost of Harrenhal
The Old Gods and the New
A Man Without Honor
The Prince of Winterfell
In the books Edit
novels, Prince Trystane is the youngest son of Prince Doran Martell, the ruler of Dorne, and third in the Martell line of succession. He has olive skin and straight black hair. He is two years older than Myrcella Baratheon, making him about 12 years old when they are betrothed.
A Song of Ice and Fire