Appearance and Character
Trystane has olive skin and straight, black hair.
Trystane suffered from redspots when he was four.
A Game of Thrones
Trystane is amongst the nobles who was summoned to submit before King Joffrey I Baratheon after he was crowned king.
A Clash of Kings
Tyrion Lannister, acting as the Hand of the King, arranges a marriage between Trystane and Myrcella Baratheon, so as to ensure House Martell would follow Joffrey. Ser Arys Oakheart accompanied Myrcella on her journey to Sunspear.
A Storm of Swords
Lord Tywin Lannister reports to Tyrion, his son, that Arys has written to him, telling him that Trystane is enchanted with Myrcella.
A Feast for Crows
Trystane and Myrcella have both learned the rules of cyvasse, and play the game together. Myrcella wins most often, but Trystane is noted to not mind. When Arianne intends to crown Myrcella, she has Arys spirit her out of Sunspear, leaving Rosamund Lannister to act as a body-double. He informs the maester that Myrcella is suffering from redspots, though Trystane still wishes to see her.
A Dance with Dragons
When Ser Balon Swann arrives at Sunspear from King's Landing, he delivers a message from Queen Regent Cersei Lannister, requesting that Myrcella and Trystane return to the capital, and that Doran come too, so as to take his place on the small council. This is revealed to be a ruse, with the true intention being for Trystane to die, and for the attack to be blamed on Tyrion, with Doran serving as a witness.