Only daughter of Queen Cersei Lannister, named for King Robert Baratheon and in truth sired on the queen by her twin Jaime, with the long Lannister gold hair and green eyes that betrayed her brothers as well. A bright, gentle child raised in the Red Keep beside her brothers Joffrey and Tommen, she was put forward by Tyrion Lannister in the second year of the War of the Five Kings as the price of Dorne's friendship, and shipped south aboard the Seaswift to be wed in time to Prince Trystane Martell, youngest son of Prince Doran. She kept her courtesies through the long voyage and the longer ride to Sunspear, and at the Water Gardens she befriended Trystane over games of cyvasse while the Sand Snakes watched and Prince Doran's plans turned slowly around her.
In 300 AC, Princess Arianne Martell, fearing herself supplanted in her father's affections and resolved to avenge Princess Elia, hatched the Queenmaker plot to crown Myrcella queen by Dornish law of primogeniture and so set the realm at war with itself. The conspirators bore Myrcella off into the desert toward the ruined holdfast of Shandystone, but Areo Hotah and Doran's guard ran them down on the road, and in the scuffle Ser Gerold Dayne, called Darkstar, struck at the princess with his longsword: the blow took off her right ear and laid her cheek open to the bone, though it had been meant to cleave her skull. Ser Arys Oakheart of the Kingsguard was slain in her defense, charging Hotah's axe in a doomed last stand, and Arianne was confined to her chambers in the Spear Tower while her father moved to contain the ruin. Prince Doran swore the maimed girl's protection, dispatched Ser Balon Swann to Sunspear with orders that ran to Darkstar's head, and sent her at last back toward King's Landing in the company of Nymeria Sand, that the queen her mother might never learn whose hand had truly held the blade.