Lord Robin Arryn is the only surviving child and heir of Lord Jon Arryn by his wife, Lysa of House Tully. He is the Lord of the Eyrie, the head of House Arryn, the Warden of the East and Lord Paramount of the Vale of Arryn. His mother was killed by Petyr Baelish in "Mockingbird" by being pushed out of the Moon Door and falling hundreds of feet to her death, leaving Petyr as the acting Lord Paramount of the Vale as Robin's step-father.
Robin was born in King's Landing where his father was serving as the Hand of the King for Robert Baratheon. Jon begins an investigation into Robert's legitimate and bastard children and then takes a fever and dies suddenly. Robin inherits Jon's position. Lysa rushes Robin back to the Eyrie following Jon's death. She writes to her sister to accuse House Lannister of poisoning Jon. Lysa was fiercely protective of Robin and he leads an extremely sheltered life.
Catelyn Stark brings the captive Tyrion Lannister to the Eyrie. Catelyn has arrested him on suspicion of involvement in the attempted assassination of her son Bran and expects Lysa's support given her earlier accusation. She is shocked to see that Lysa still breastfeeds Robin, despite him being so old. Robin is prone to fits of histrionics, demanding to see "the bad man fly". Lysa calls him a strong and mighty ruler, worthy of the Vale's leadership, but Catelyn appears skeptical.
Tyrion agrees to confess to his crimes and is brought before the High Seat. He presents a list of minor misdemeanors from childhood, though Robin is particularly intrigued by some of them. Tyrion demands a trial by combat to prove his innocence and persuades the mercenary Bronn to be his champion. Bronn triumphs against Robin's champion, Ser Vardis Egen and he and Tyrion are allowed to go free. Robin is disappointed at being deprived of an execution by Moon Door.
Catelyn visits Robin and Lysa before leaving the Vale to join her son Robb's army. She asks Lysa to commit the Vale to the war on the side of the Starks and Tullys, but Lysa refuses, saying the Vale's knights are needed at home to defend Robin.
Robin receives his new step-father, Petyr Baelish, and his cousin Sansa Stark at the Eyrie. Robin takes temporary delight in the gift Petyr brings him - a falcon, finely carved from crystal - before throwing it out the Moon Door to demonstrate the Door's nature to Sansa. Lysa seems to have allowed him some freedom since Littlefinger's arrival, as she allowed him out of her sight to show Sansa around.
Later, he visits Sansa in the courtyard of the Eyrie while she is building a snow castle of Winterfell. When he suggests they add a Moon Door to it, he accidentally knocks over one of the towers that Sansa has built. At first, she gently scolds him, but as he grows petulant, insisting that he didn't ruin it, Sansa is driven to slap him across the face. He then runs back into the castle. Later on, his mother is killed by being pushed out of Robin's favorite plaything, the Moon Door, by Petyr Baelish.
Robin begins sparring lessons, under the supervision of Lord Yohn Royce. Due to his frail condition and sheltered childhood, Robin's attempts at swordsmanship are quite pitiful for a boy of his age. Petyr Baelish and Sansa Stark leave Robin behind to be fostered at Runestone under Royce's tutelage, who assures them of their Lord's safety, though he makes no promises that Robin will become a swordsman.
Robin is practicing archery at Runestone under the tutelage of Lord Yohn Royce. He is excited upon the arrival of Petyr Baelish, who he refers to as Uncle Petyr, and is captivated by his present of a falcon. He offers to send Lord Yohn flying for treason, but allows him leniency upon Petyr's recommendation. He decides that the Vale armies should go north to defend his cousin, Sansa Stark, from House Bolton.
Robin Arryn lived an extremely sheltered life in the Eyrie under his mother's watchful eye. Now orphaned, Robin was left behind at Runestone to foster with Lord Royce while Littlefinger traveled with Sansa. Upon the return of his "uncle" Petyr, Robin pledged the Knight's of the Vale to ride forth and help Sansa.