Myrcella Baratheon
The princess of the realm, sweet-natured and biddable, the middle of Cersei's three children.
The fuller tale
Myrcella is the middle child and the only one of Cersei's three the court never worried about. Sweet-natured where Joffrey is vicious and confident where Tommen is timid, she is by every account — including her uncle Tyrion's, which is the one that counts — the child who got the family's beauty and brains without its poison. The books toss off the observation that makes her poignant: had she been born a boy, the realm might have been fine. Instead she is born a princess, which in Westeros means she is a treaty waiting to be signed.
Tyrion signs it. As acting Hand during the war, he ships her to Dorne — over Cersei's white-hot objection — as the intended bride of Trystane Martell, reasoning that she is safer in Sunspear than in a besieged capital, and the farewell at the docks buys him Cersei's promise that this will be paid for. Dorne agrees with her: the show returns to her seasons later in the Water Gardens as a happy young woman genuinely in love with her betrothed, politely uninterested in being rescued from her own life. When Jaime arrives on his ill-conceived extraction mission, she tells him the truth nobody in her family can ever quite manage — she knows where she is wanted.
Her last scene with Jaime is the family's one unpoisoned moment. On the deck of the ship home, she tells him she knows he is her father — has known for a while — and that she is glad. It is the single instance in the entire saga of a Lannister secret coming out and making someone happy, and the show hands it to the gentlest of the golden children, which is exactly why it cannot be allowed to last.
The page and the screen
The books send her to Dorne on the same logic and into a much more tangled story. There she becomes the centerpiece of Arianne Martell's queenmaking plot — a Dornish scheme to crown Myrcella under Dornish law, which recognizes the eldest child regardless of sex — and when the plot unravels, the knight called Darkstar turns his blade on the child herself; book Myrcella survives him, but maimed, losing an ear and carrying the scar across her cheek. None of it — Arianne, the queenmaking, Darkstar — reaches the show, whose slimmed-down Dorne plot substitutes the Sand Snakes' vengeance for the books' constitutional crisis. She also gets a different confidence: the books have not given her the paternity scene with Jaime, which is the show's own invention and arguably its best addition to her.
Where they stand, season by season
Their fate — if death has found them, it is written here. Open at your peril.
The show: Poisoned in Dorne (S5·E10)
Ellaria Sand kissed the princess farewell at Sunspear's docks with poison on her lips. Myrcella died at sea in her father's arms — moments after telling Jaime she knew, and was glad.
In the show she dies of a kiss. Ellaria Sand, unappeased by peace with the Lannisters, bids the princess farewell at Sunspear's docks with poison on her lips — the long farewell, an antidote taken by the poisoner alone — and Myrcella sails for home apparently well. The venom works on the water: moments after telling Jaime she knows he is her father and is glad of it, she bleeds from the nose and dies in his arms, the second golden shroud of Maggy's prophecy delivered mid-embrace. Ellaria's price is paid later, when Cersei returns the exact kiss to her daughter Tyene in the black cells. In the published books Myrcella is alive — scarred by Darkstar's blade during the collapse of Arianne's plot, short an ear, but breathing, and on her way back into the story's center as Dorne weighs its next move.