House Manderly of White Harbor is the wealthiest and most southerly of the houses sworn to Winterfell. They are Reach folk by blood, exiled from their seat at Dunstonbury a thousand years before the Conquest after a quarrel with King Perceon III Gardener, and given the mouth of the White Knife by King Jon Stark on condition that they swore him their swords forever. They did, and they keep.
Alone among northern lords the Manderlys still revere the Seven, still honor knighthood, and still maintain a true city, White Harbor itself, the only proper city north of the Neck. Its New Castle rises white above the harbor, the Wolf's Den at the river's mouth has been their dungeon since they took the keys from House Greystark, and a Sept-of-the-Snows shines on the hill behind the wharves where merchants from Braavos and the Free Cities pay tariff in silver moons.
By the events of A Game of Thrones the house is led by Lord Wyman Manderly, called by some Lord Too-Fat-to-Sit-a-Horse, enormously fat, enormously rich, and enormously loyal to the Starks who gave his forebears their second life. His eldest son Ser Wylis, heir to White Harbor, was taken captive at the Battle of the Green Fork; his second, Ser Wendel, rode south with Robb Stark's host and was slain at the Red Wedding with a crossbow bolt through the brain. Wyman's granddaughters Wynafryd and Wylla wait at the New Castle for grandfather to choose what kind of vengeance an old fat man may yet deliver.