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."
After the taking the black and becoming Lord Commander, he went missing on a ranging mission and was never heard from again. The show has never outright confirmed that Brynden is the Three-Eyed Raven, but the books heavily imply it, and most fans consider it canon. How, exactly, he became Three-Eyed Raven is a mystery, but he's said to have the gift of greensight, which means he has prophetic dreams -- just like Bran. It is unclear if he was the very first Three-Eyed Raven, but it seems likely. He lived among the Children of the Forest, who are also greenseers, and who may have used their own magic to anoint him the role.
On the show, Brynden teaches Bran how to us his second sight to plug into and witness history, but stresses that he must not change it -- a rule Bran accidentally breaks when he wargs into past-Hodor and scrambles his brain. Brynden is eventually killed by the Night King, which makes Bran -- the "last greenseer" -- the Three-Eyed Raven he was destined to become.