Trystane is the youngest son of Prince Doran Martell.
Prince Trystane is a member of House Martell, the ruling house of Dorne. He is the son of Prince Doran Martell, Lord of Sunspear and Prince 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
Trystane is briefly seen walking and flirting with his betrothed Myrcella Baratheon in the Water Gardens during the argument between Ellaria Sand and Prince Doran Martell.
When Jaime Lannister and Bronn enter the Water Gardens, Trystane is again taking a walk with Myrcella, with Trystane suggesting they should ask Doran to have them married the next day. Myrcella asks if he only wants to marry her because they are both from Great Houses, but Trystane cuts her off with a passionate kiss. Doran, observing the happy couple, muses to Areo Hotah that they don't realize how much danger they are in and that they must be protected.
While kissing Myrcella, Trystane encounters Bronn and Jaime in the Gardens. Although he introduces himself courteously enough, he is clearly suspicious, having heard no news of Jaime's visit to Dorne; his suspicions are aroused further when he sees bloodstains on the stolen robes Jaime and Bronn are wearing. Although Bronn cautions him not to do something stupid, Trystane reaches for his sword, forcing Bronn to knock him out, to Myrcella's distress. Jaime then attempts to take Myrcella out of the palace, but he and Bronn are then attacked by Trystane's cousins, the Sand Snakes. The skirmish ends when all five of the intruders are arrested by Areo Hotah and his guards.
Doran invites Trystane, Myrcella, Ellaria and Jaime to drinking and eat with him. There, he promises Jaime that he does not want a war with the Lannisters and that he will send Myrcella back to King's Landing. However, Trystane and Myrcella's betrothal is to stand, which means Trystane will accompany his betrothed to the capital. He will also take up Oberyn's vacant seat on the Small Council, to further cement the alliance between Dorne and the Iron Throne. Jaime accepts these terms.
Jaime then brings up the matter of Bronn, who is still imprisoned as a result of his aggression on the prince. Doran defers to Trystane to decide, as he must learn how to punish lawbreakers if he must one day rule. Trystane agrees to let Bronn go on one condition: he will receive the same thing that he inflicted upon him. This meets with his father's approval and Bronn is brought before them and Areo elbows him hard across the face. Trystane later accompanies Jaime, Myrcella and Bronn to the harbor of Sunspear, where he remains silent, as the others say farewell to Prince Doran and Ellaria.
Apparently to ensure Trystane's safety, Jaime leaves him on board the ship once they reach King's Landing. As Trystane paints the stones for Myrcella's funeral, he is accosted by Obara and Nymeria, who have been sent to kill him by Ellaria, who has herself murdered Doran in Dorne. Trystane attempts to reason with his cousins that they are family, but they ignore him. He is given a choice to fight Obara or Nymeria. Though he draws his sword and prepares to fight Nymeria, he is impaled through the back of the head by Obara and killed instantly, much to Nymeria's chagrin.
A prince of Dorne and heir to Doran Martell, Trystane was engaged to Myrcella Baratheon, whom he genuinely loved. Myrcella died on the voyage to King's landing, poisoned by Ellaria Sand in revenge for the death of her lover Oberyn. Unable to set foot in the capital, Trystane remained on the ship - where he was ambushed and killed by the Sand Snakes.