Trystane Martell
Doran's gentle youngest son, betrothed to Princess Myrcella.
The fuller tale
Trystane is Doran's youngest, and both tellings agree on the essentials while disagreeing about nearly everything else: a gentle, well-made boy, raised at Sunspear far from the realm's wars, and betrothed to Myrcella Baratheon as the seal on Dorne's peace with the Iron Throne. The books keep him a child at the Water Gardens — eleven or so, beating his betrothed's guards at cyvasse because Myrcella coaches him, a small warm corner of a plot otherwise made of knives — while the show ages him into a young man in love, all courtesy and courage without practice, walking hand in hand with his princess through gardens his uncle's widow is measuring for a coup.
The show hands him one moment of unexpected steel: when the Sand Snakes' plot first brushes Myrcella, it is Trystane who stands between, and when Doran offers Jaime Lannister a settlement, Trystane is its living term — a seat on the small council, a future spanning both courts, the marriage made policy. He sails for King's Landing as the peace's whole cargo, which in this story is the most dangerous office there is.
The page and the screen
Book Trystane is barely more than a warm detail — the cyvasse games with Myrcella are his entire on-page career, and he remains at the Water Gardens, alive, a minor piece in his father's long game with his siblings carrying the plot's weight. The show, having cut Arianne and Quentyn, promoted Trystane to only child and gave him their combined narrative exposure with none of their protection: the romance, the council seat, and the voyage north are all invention, and so is everything the voyage costs. The books' Dorne guards its children better — so far.
Where they stand, season by season
Their fate — if death has found them, it is written here. Open at your peril.
The show: Slain aboard ship (alive in the books) (S6·E1)
Doran's gentle son was murdered aboard ship in Blackwater Bay by his own cousins, a spear through the back of the head while he painted funeral stones — the Sand Snakes' coup made complete.
His death is show-only, and it completes the coup. Aboard ship in Blackwater Bay — painting the funeral stones for Myrcella, the eyes that Westerosi custom lays on the dead — Trystane is cornered by his cousins Obara and Nymeria, come from Sunspear with his father's blood already on the family's hands. He is offered a choice of killers and given no real chance against either; Obara's spear takes him through the back of the head mid-turn, a murder even the murderers seem faintly embarrassed by. With Doran and Trystane both gone, the show's Dorne passes entire to Ellaria and the Sand Snakes, the peace he embodied dying with him. In the published books Trystane Martell is alive at the Water Gardens, still losing gracefully at cyvasse, his betrothed maimed but breathing in the rooms nearby.