Only son and heir of Lord Jon Arryn and his third wife Lysa Tully, born sickly in King's Landing while his father was Hand of the King and named for the king himself. He is six years old at the events of A Game of Thrones but small and painfully thin for his age, still suckling at his mother's breast, given to falling fits the Vale maesters cannot master and a tongue that goes loose with weeping at any thwart. The realm calls him Lord Robert; his mother calls him Sweetrobin.
When Lord Stannis Baratheon was named to foster the boy at Dragonstone, Lysa fled the capital with him for the Eyrie and shortly thereafter, by Petyr Baelish's hand, poisoned her husband to keep her son. From the high seat of the Vale she rules in Robert's name and lets him cast accused men through the Moon Door for sport. After her own death from the same Door at Littlefinger's push, the boy passes into Lord Petyr's keeping at the Gates of the Moon. Across A Feast for Crows and A Dance with Dragons Petyr quietly weans him from the breast, doses him with sweetmilk for the shaking, and steers his betrothal toward Ser Harrold Hardyng, the named heir behind him, a long patient design whose end none but Lord Baelish yet knows.