The Brotherhood is an outlaw organisation founded by Beric Dondarrion to bring justice to the Riverlands after it is attacked by the Lannisters. It becomes a center for war after Robb Stark arrives in the Riverlands, and after his father Eddard Stark's death at the order of the Lannisters' King Joffrey Baratheon, being proclaimed King in the North.
When Eddard's younger daughter Arya Stark and a bastard of Robert Baratheon, Gendry, are taken to the Inn of the Kneeling Man, they encounter Greenbeard and other members of the Brotherhood. Greenbeard leads the group which escorts Arya and Gendry to Beric, often referring to Arya as a squirrel. When he hears that Ser Jaime Lannister has escaped Riverrun who says the Red Priest Thoros of Myr must be told. He asks the Ghost of High Heart if she has heard word of Beric, and hears he was there not a fortnight past.
Another member of the Brotherhood, the Mad Huntsman, catches Sandor Clegane "the Hound", whose brother Ser Gregor Clegane, "the Mountain that Rides", is among the Lannisters' forces. Before he can put Sandor in a crow cage the Brotherhood make him agree to bring Sandor before Beric, who is in the Hollow Hill. In a trial by battle Sandor defeats Beric. The Hound is released though his gold is confiscated. With the gold Greenbeard is sent to buy grain and seed south of the Mander, accompanied by the Mad Huntsman.
- It is possible that Greenbeard is a deserter from the Tyroshi sellswords fighting in the War of the Five Kings. George Martin claimed there is at least one Tyroshi deserter in the Riverlands, though he couldn't say whether they deserted Jaime or Robb.