The original Three-Eyed Raven
Bran started having visions of the raven shortly after losing the use of his legs. In his dreams, he was summoned by the mythical bird, and after escaping a Theon-ruled Winterfell, he met Jojen and Meera, who pledged to unite him with the raven. After a long journey, and Jojen’s death, Bran finally meets a very old man who lives in -- and partially is -- a tree. The man tells Bran he'll never walk again, but that he will fly.
In the books, it is suggested that this man is Brynden Rivers, the legitimized bastard son of King Aegon IV Targaryen. In his former life, Brynden fought in the Blackfyre Rebellions, served as hand of the king to Aerys I and Maekar I Targaryen, and, eventually, became Lord Commander of the Night’s Watch. Rivers was an albino with a large red birthmark on his face that earned him the nickname "Bloodraven." Rumor had it that he was a sorcerer, and that he ruled the kingdom with "spies and spells."