Spoiler warning! The following article contains major plot points from Game of Thrones season six, episode five.
Although we don't meet the Three-Eyed Raven on Game of Thrones until season five, his presence hangs over the entire series. When Bran falls from the tower and slips into a coma, the Three-Eyed Raven appears to him as a bird infiltrating his dreams. Later, Bran hears his voice coming from a weirwood tree, directing him to go north. When Bran finally meets the Three-Eyed Raven in person, he's an old man enfolded in the branches of a weirwood tree. When the Three-Eyed Raven dies defending Bran from the White Walkers on the latest episode, he leaves both Bran and the audience with more questions than answers.
Who Is He?
Nobody seems to be sure who exactly the Three-Eyed Raven is. Unlike the hundreds of other characters populating the Game of Thrones universe, he doesn't have a name. He doesn't have a house affiliation. In the Song of Ice and Fire books, his black cloak hints at a previous association with the Night's Watch, but he never talks about his own history.
What we do know for sure about the Three-Eyed Raven is that he is a Greenseer, gifted with the ability to see the past, present, and future and communicate through dreams. Greensight is more common with the Children of the Forest, but some humans, like the Three-Eyed Raven, also have this ability.
What Does He Want?
When Bran meets the Three-Eyed Raven, he says he's waited a thousand years for him. He assumes a mentor role to Bran, showing him visions but stressing that he cannot interact with the past. The Three-Eyed Raven assures Bran he won't become an old man stuck in a tree. So what exactly are his plans for Bran? The Three-Eyed Raven isn't a passive observer. After all, he uses his powers to bring Bran north. But he seems adamant that Bran shouldn't fiddle with the past.
What Does His Death Mean For Bran?
The Three-Eyed Raven tells Bran that he now must become the Three-Eyed Raven himself, even if he isn't ready. We've seen that Bran can affect the past, as his manipulation of Hodor's mind to defend himself from the White Walkers leads to Hodor's mental problems and eventual death. Will Bran heed the Three-Eyed Raven's warning about messing with past, or will he continue to explore his newfound abilities? We may not see the Three-Eyed Raven again, but his vision training with Bran will have enormous implications going forward.