Princess Myrcella is commonly thought to be the only daughter of King Robert Baratheon and Queen Cersei Lannister. Like her siblings, her real father is Jaime Lannister. Her younger brother Tommen Baratheon currently sits on the Iron Throne as the King of the Andals and the First Men. She has been sent to Dorne as part of a marriage pact with House Martell.
Princess Myrcella is commonly thought to be the only daughter of King Robert Baratheon and Queen Cersei Lannister. Cersei and Robert married to seal a political alliance between House Baratheon and House Lannister after Robert took the throne by force from the Mad King Aerys II Targaryen. Myrcella has an older brother, Joffrey, and a younger, Tommen, with whom she is close. The real father of all three children is Jaime Lannister, making them bastards born of incest. However, like her siblings and the rest of Westeros, Myrcella has no idea of the truth of her parentage. She has a sweet and quiet disposition and is fond of her uncle Tyrion Lannister.
Myrcella accompanies her family north to Winterfell after the death of Jon Arryn. During their stay Bran Stark is severely injured. Myrcella is eating breakfast with Cersei, Jaime and Tommen when her uncle Tyrion joins them. Myrcella asks whether Bran will die and Tyrion replies that he is expected to live, delighting Myrcella and horrifying Cersei. Tyrion excites Myrcella and Tommen by announcing that he wishes to see the Wall and the “wintry abode of the White Walkers.” He then amuses them by claiming that he wants to urinate off the edge of the wall. Cersei is irritated by his rudeness and leaves, taking the children with her.
Myrcella attends the tournament to celebrate Eddard Stark becoming Hand of the King. She witnesses the horrific death of Ser Hugh of the Vale during his joust against Ser Gregor Clegane. Myrcella returns for the second day of the tournament and applauds when Ser Loras Tyrell unseats Gregor. Gregor attacks Loras and Sandor Clegane intervenes. King Robert calls an end to the fighting. Myrcella joins the applause when Loras declares Sandor a champion.
Myrcella attends Joffrey's nameday Tourney. She is pleased when her uncle Tyrion Lannister arrives during the festivities and tells her that she is more beautiful than ever. When Joffrey callously says that they had heard he was dead, Myrcella tells Tyrion that she is glad that rumors of his death were not true.
Myrcella attends a tense dinner with her younger brother Tommen, her mother Cersei, and Sansa Stark. Myrcella naively makes smalltalk with Sansa, asking when she will marry Joffrey, and Cersei explains it will be after the war ends. Myrcella says that she is excited for the new gowns she will wear to the wedding. She good-naturedly points out that Sansa will be wearing an ivory gown as the bride. Sansa isn't sure how to react but Cersei presses her to respond. Sansa parrots the usual lines that she looks forward to being married to Joffrey.
In order to strengthen House Lannister's political position during the War of the Five Kings, Hand of the King Tyrion Lannister makes plans to forge a marriage-alliance with House Martell of Dorne by having Princess Myrcella wed to Trystane Martell, the youngest child of the current ruler of Dorne. In return House Martell would hopefully supply soldiers to the Lannisters' side, or at least ensure they wouldn't openly side with their enemies.
The actual marriage ceremony will wait until Myrcella is old enough, but part of the arrangement involves sending her to Dorne to live in Trystane's household. Queen Cersei is furious with Tyrion when she hears (from Grand Maester Pycelle) what he is planning, declaring that she won't let her only daughter be shipped off to Dorne as she was shipped off to Robert Baratheon. Tyrion counters that as a princess, many would say that Myrcella was born to one day forge a marriage alliance, and she will be safer in Dorne than in the capital city during a major civil war. Against Cersei's continued protests, Tyrion insists that the plan has already been finalized, leaving Cersei fuming and in tears for her daughter.
Myrcella later plays with her handmaidens in a castle garden before the eyes of Septa Eglantine, while her inebriated mother bitterly criticizes Tyrion for being ready to "seal my daughter in a crate so you can ship her away". Tyrion insists that she will be safer in Dorne, and Cersei mocks his concern for her. Tyrion firmly says that he is indeed concerned for Myrcella's safety: she is a sweet, innocent girl, and "I don't blame her at all for you". Myrcella tearfully departs King's Landing on a ship bound for Sunspear accompanied by Septa Eglantine and Ser Arys Oakheart of the Kingsguard. She is seen off at the docks by a formal royal procession including King Joffrey, Prince Tommen, Queen Cersei, Acting Hand of the King Tyrion Lannister, and Sansa Stark with prayers for her and her safe journey delivered by the High Septon.
After Jaime returns to King's Landing, Cersei lists off to him all she has suffered since he left for the war over two years ago, one of which is that their daughter Myrcella has been shipped off to Dorne to secure a political marriage-alliance.
During the royal wedding between King Joffrey and Margaery Tyrell, Prince Oberyn Martell encounters Lord Tywin, the man he holds responsible for letting Ser Gregor Clegane rape and kill his sister Elia during the Sack of King's Landing, and also killing her two children. Oberyn introduces his paramour, Ellaria Sand, to Myrcella's mother Cersei. The four of them exchange smiling pleasantries which barely hide their underlying and mutual loathing for each other. At the end of their encounter, Oberyn casually points out that Myrcella is still in Dorne; while Oberyn hints that a ward can easily become nothing more than a hostage, he also taunts Cersei and Tywin by stating that murdering little girls is frowned upon in Dorne. (The obvious implication is that Oberyn and his family are morally superior to the Lannisters and unwilling to hurt Myrcella for a political advantage despite the Lannisters killing House Martell's relatives for their own.) Tommen is crowned king after Joffrey is poisoned at his own wedding, making Myrcella the heir to the Iron Throne.
While conversing with Oberyn, Cersei laments that she has not seen Myrcella in over a year. Oberyn informs her that the last time he saw Myrcella she was swimming with two of his daughters in the Water Gardens, laughing in the sun. Cersei states that she wants to believe this story, and that Myrcella is happy. Oberyn gives her his word, stating again that they do not hurt little girls in Dorne. Cersei replies that they hurt little girls everywhere in the world. Cersei asks Oberyn to bring Myrcella a gift from her, as she missed her nameday and does not know when she will see her again. Oberyn agrees and Cersei points to a ship that the best shipwrights in King's Landing have been building for months, declaring that Myrcella loves the open water. Oberyn promises to have it sailed down to Sunspear for Myrcella. Cersei asks him to please tell Myrcella that her mother misses her very much. Later, Oberyn meets with Tyrion in his prison cell, and recounts his meeting with Cersei. Oberyn says he half-suspects that Cersei was actually lying, and was exaggerating how much she misses Myrcella in order to guilt him into siding with her against Tyrion during the trial - though her performance was so convincing that part of him also wonders if it was true. Tyrion explains that the trouble with Cersei is that she mixes lies with truth to suit her own ends: part of her is worried about Myrcella, but she isn't above cynically using this grief to gain sympathy from others.
Myrcella walks through the Water Gardens with her betrothed Trystane Martell, whom she has fallen in love with, while being watched by Ellaria Sand and Prince Doran Martell. Ellaria, still furious about Oberyn's death, offers to torture and mutilate Myrcella, and send pieces of her back to Cersei as a message, but Doran staunchly refuses to harm the girl.
As Myrcella and Trystane are wandering the water gardens, Myrcella questions Trystane as to if he truly loves her or is only indifferent towards their arranged pairing. Trystane affirms that he has true affections for her. They are once again observed by Prince Doran, who muses to his guard Areo Hotah that a Lannister and Martell pairing is indeed very dangerous.
Myrcella is suprised to see her "uncle" Jaime Lannister after he and Bronn finally locate her as she is kissing Trystane in the gardens. She is upset after Bronn is forced to knock her betrothed unconscious, and struggles against Jaime's advances to leave with him. After the Sand Snakes make a sudden ambush in order to aprehend Jaime, Myrcella is nearly taken hostage by Nymeria Sand, but the conflict is mediated after Areo Hotah arrives with a dozen Dornish guards, imprisoning Jaime, Bronn and the Sand Snakes.
Myrcella visits Jaime in custody. Jaime tells her that the situation has grown dangerous and he needs to take her home, back to King's Landing. Myrcella states that Dorne is her home now, and she'll stay and marry Trystane before leaving Jaime alone.
Myrcella is then seen seated next to Trystane at the meeting between the Martells and Jaime. When her 'uncle' remarks that she must be cold in her dress, implying he doesn't approve of the more revealing style of Dornish clothing, Myrcella cooly replies that she is not and that the warmer climate agrees with her, thereby upholding and reasserting her commitment to Dorne and her new life and also demonstrating how much she has flourished there. When Jaime states that her brother Tommen wishes her to return to King's Landing, she is both nonplussed and bothered, but she is palliated when Doran calls for her engagement to his son to continue and that he joins her in the Capital, a request Jaime agrees to.
When Ellaria later visits Jaime she reveals that she knows Myrcella is his daughter, born of incest with Cersei. Ellaria admits that she is aware of Myrcella's innocence and that she is not affiliated with Oberyn's murder in an apologetic manner.
At the harbor, Myrcella says her farewells to Prince Doran and Ellaria before departing for King's Landing. Ellaria, taking her vengeance, kisses her after coating a fatal poison on her lips, thus sealing the young girl's fate. On board the boat, Myrcella reveals to Jaime that she has always known of his and Cersei's incestuous relationship, and that he is her true father. The two of them share a tender moment, before the effects of the poison become apparent. Jaime holds Myrcella as she falls, blood running from her nose.
Cersei's only daughter, Myrcella was sent to Dorne as part of a deal her uncle Tyrion struck with House Martell to align their houses and keep the princess safe. Betrothed to Trystane Martell, son of Prince Doran, she was poisened by Ellaria Sand who wished to avenge the death of her lover Oberyn Martell. Myrcella died in the arms of Jaime Lannister, whom she acknowledged as her father before her death.