Lysa was the Lady Regent of the Vale, ruling on behalf of her young son Robin Arryn. She doted on her son and pursued a policy of isolationism since the death of her husband Jon Arryn. She presided over the trial of Tyrion Lannister for the murder of Jon Arryn and the attempted murder of Bran Stark. He proves his innocence in a trial by combat. Lysa refused to commit her forces to aid her sister Catelyn Stark in the War of the Five Kings.
Lysa was born into House Tully and was the second child of Lord Hoster Tully. She had an older sister, Catelyn and a younger brother, Edmure. Petyr Baelish was fostered with the Tullys and Lysa grew up with him.
Lysa married Jon Arryn during Robert's Rebellion. Lord Jon was the head of House Arryn and a leader of the rebel faction fighting to overthrow the Mad King. The rebels forged an alliance with House Tully through marriage, with Lord Eddard Stark married to Catelyn and the then-elderly Jon to the still young Lysa.
After the rebel victory in the war, Robert Baratheon became King and appointed Jon as his Hand. Jon served the realm well for many years as Hand of the King. Lysa and Jon have a single son, Robin Arryn. Lysa and Jon spent the next seventeen years in the capital of King's Landing, never returning to the Eyrie in the Vale of Arryn.
Jon died immediately prior to the events of the series, sparking King Robert's search for a replacement. Lysa fled King's Landing for the safety of the Eyrie with Robin, then six-years old. Robin is the titular Lord Paramount of the Vale. However, Lysa will rule as the Lady Regent of the Vale until Robin comes of age. She dotes on her son. She is considered mentally unstable by Tyrion Lannister.
Lysa's husband Jon Arryn, the Hand of the King, dies of an apparent fever. Lysa flees the capital and returns to the Eyrie with her son Robin. She sends her sister Catelyn a letter claiming that the Lannisters murdered Jon Arryn for unknown reasons. This news convinces Catelyn's husband, Lord Eddard Stark, that he must accept King Robert's offer to become the new Hand so he may investigate further. During the king's visit to Winterfell Catelyn's own son Bran Stark is injured in suspicious circumstances.
Catelyn apprehends Tyrion Lannister on suspicion of trying to kill Bran. She brings him to the Eyrie to be questioned. Lysa is furious with Catelyn for exposing her to the wrath of House Lannister. She and Robin hold Tyrion in contempt, ignoring his threats that his house will repay them for any harm they visit upon him. Lysa orders Tyrion sent to Mord, the castle jailer. Tyrion and Catelyn are both disturbed by Lysa's excessive coddling of Robin, including still breast-feeding him despite him being several years too old for it.
Tyrion convinces Lysa to give him a trial, only to protest his innocence. He demands the right of trial by combat, supported by the mercenary Bronn. Bronn wins the battle, killing Lysa's champion, Ser Vardis Egen. Lysa chides Bronn for not fighting with honor, but reluctantly releases Tyrion.
Lysa receives a letter from King's Landing announcing that Robert has died and Eddard has been arrested as a traitor. She delays showing the letter to Catelyn, to Catelyn's annoyance. Catelyn asks that the knights of the Vale join House Tully and House Stark in fighting the Lannisters for his freedom, but Lysa refuses. Lysa insists that the Vale knights must remain at home to defend Robin. Catelyn leaves, frustrated and annoyed.
Petyr Baelish, acting under his own promises to both Lysa and her niece Sansa Stark, arrives at the Vale, bringing Sansa to the Eyrie with him. Lysa greets Sansa enthusiastically, introducing her to her son Robin. She conveys her pity for Sansa's unfortunate marriage with the dwarf Tyrion Lannister, and asks if he ever forced himself on her, to which she replies "no". Lysa agrees not to refer to Sansa by her true name in the presence of anyone else, and has Robin show her around the castle. Sexually eager, Lysa has her and Littlefinger wed immediately, promising to "scream" as he makes love to her. She also references poisoning her own husband's wine and framing the Lannisters in his presence, using these actions as proof of her love for him
The following day, Lysa dotes over Sansa in petty conversation, and she shares her childhood memories about Catelyn. However, she grows angry when Sansa almost mentions that Littlefinger loved Catelyn, something which has irked Lysa for years. She starts grilling Sansa, wondering why Littlefinger cares so much for her, and if he has ever forced himself onto her. Sansa is shocked and denies this, and complains that Lysa, clenching her fingers tightly, is hurting her. Seeing Sansa scared, Lysa returns to a nurturing state, drawing her close and comforting her.
Lysa witnesses Littlefinger kissing Sansa in the Eyrie's gardens, and goes mad with jealousy. She summons Sansa to the High Hall of the Arryns, threatening to push her out the Moon Door to her death, but is interrupted by Petyr.Emotionally distraught and torn over Littlefinger's infatuation with Sansa, Lysa falls into a fit of tears, shoving her to the floor. Littlefinger feigns an apology, embraces her and reassures her that he only ever loved one woman. To Lysa's horror, he utters "your sister", and then shoves Lysa through the Moon Door to her death, as a shell-shocked Sansa looks on.
Catelyn Stark's younger sister, Lysa was married off to the much older Jon Arryn (the former Hand of the King) when Robert Baratheon led his rebellion against the Mad King Aerys. After her husband's unexpected death, Lysa fled to the Eyrie, where she kept a careful eye on her son Robin and avoided the war below. She eagerly married her childhood love, Petyr Baelish - who courted her to secure the region for the kingdom - but he killed her when she threatened her niece Sansa.