House Frey of the Crossing is the youngest of the great Riverlands houses, and the wealthiest after the Tullys themselves. Six hundred years ago they were petty knights with a stretch of bog beside the Green Fork; today they hold the only bridge across that river for many leagues, two great stone towers joined by a covered span, and a Frey toll has been paid by every army, merchant, and lord who would cross between the Trident and the North.
The Freys had their letters of patent and their bridge tolls from House Tully, and in name they have answered to Riverrun ever since; in practice they have made themselves a small kingdom of cousins. Each generation has added another wing to the Twins, another marriage match into the riverlands, another keep along the Green Fork, until no other bannerman of Riverrun can muster half so many lances. Their loyalty to their liege has been steady but seldom hurried; the Freys have a name for arriving late wherever blood is spilled, and for remembering at length any slight, however small.
Lord Walder Frey is past ninety by the time of A Game of Thrones, on his eighth wife, with so many sons and grandsons and great-grandsons that the maester at the Twins keeps a written list to remember them by. His liege Lord Hoster Tully has been heard, in private, to call him the Late Lord Frey, for arriving on the field at the Trident only after the battle was won, a sobriquet Walder is said to know and to nurse, against the day a Tully might need him most.