Heir to House Selmy of Harvest Hall in the stormlands, who took up sword and saddle at the age of ten and rode into the lists at Blackhaven in mismatched armor under the name Ser Barristan the Bold, where he was unhorsed by the young Prince Duncan Targaryen. The name stuck, and the boy who wore it grew into the foremost knight of his age: champion of the tourney at King's Landing in 257 AC, slayer of Maelys the Monstrous in single combat upon the Stepstones during the War of the Ninepenny Kings, and at sixteen the youngest man ever raised to the Kingsguard by King Jaehaerys II for that deed.
He wore the white cloak through three reigns. Under Aerys II he rode in the vanguard at the Battle of the Bells, where he was wounded thrice and cut down many a stormlander before being unhorsed in the streets of Stoney Sept; at the Trident he stood by Prince Rhaegar to the last and was struck down by Robert Baratheon's hammer, only to be pardoned and named Lord Commander of the Kingsguard by the new king upon waking in his sickbed. For seventeen years he served the usurper as faithfully as ever he had Aerys, and rode in the Greyjoy Rebellion at Robert's side.
His service ended in 299 AC when the boy king Joffrey, at Queen Cersei's urging, stripped him of his cloak in open court on the pretext of his age and sent him from the Red Keep with nothing but the rebuke "old man." Ser Barristan put off his white armor, slipped from the city under a false name, and made his way east to Pentos and beyond, taking service as Arstan Whitebeard in the train of Daenerys Targaryen, daughter of the last true king he had sworn to serve. When he revealed himself to her at Meereen she forgave him his deception, and he became her Lord Commander of the Queensguard, and after her flight upon Drogon into the Dothraki sea was named Hand of the Queen in the city she left behind.