Duncan's origins are, by design as much as by loss of record, close to a blank page: raised in King's Landing's Flea Bottom, of a birth low enough that no chronicler thought it worth preserving even after he'd risen far past it. He came up squiring for an aging hedge knight, Ser Arlan of Pennytree, and inherited the man's sword, his shield, and his wandering trade when Arlan died on the road — acquiring, along the way, a squire of his own: a secretive boy called Egg, who turned out to be Prince Aegon Targaryen, fourth son of a fourth son and, by the accidents of the Great Council of 233 AC, eventual king.
That friendship carried Duncan further than his birth ever should have. When Egg became Aegon V, Duncan followed him into the Kingsguard and, in time, commanded it — a hedge knight without a household name leading the seven sworn to the body of the King of the Andals, the Rhoynar, and the First Men, which the Citadel's own records treat as remarkable enough to note twice.
He died, so far as the surviving accounts agree, at Summerhall in 259 AC, in the fire that also killed Aegon V and the king's own son and namesake, Prince Duncan the Small — a blaze connected, by every scrap of testimony that survived it, to the king's obsession with hatching dragon eggs by ritual and wildfire together. What exactly happened in the castle that night the Citadel cannot say with confidence; the maester who might have explained it left only a letter so badly blotted by rain that its second half is unreadable.