Jaqen H'ghar is a faceless man who first appears in A Clash of Kings, when Arya Stark heads down the Kingsroad in the care of Yoren, a brother of the Night's Watch. Jaqen is heading for the Wall with two other captives named Rorge and Biter. He tries to talk to Arya, but she manages to avoid him until those bound for the Wall are attacked by Lannister men who kill Yoren and set the barn that Jaqen and his companions are tied up in on fire. Arya gives Jaqen an axe to set them free, and then escapes. She makes her way to Harrenhal, currently under the care of Lannister men, and goes to work there.
Arya meets Jaqen again when he becomes a member of the Lannister men at Harrenhal. There, he tells her that he owes her three deaths in exchange for her saving him, Rorge, and Biter. She chooses for him to kill two men working under the Lannisters, and then tricks him into helping her free Roose Bolton's men so that they can take over Harrenhal. After this, Jaqen H'ghar reveals his faceless nature to her, gives her a coin, and tells her that she can find him with the coin and the words valar morghulis. He then shape-shifts into another man.
Initially, Jaqen is a young man with longish hair that is white on one side and red on the other. He speaks in a unique accent, and always in third person. He heavily influenced Arya, so much so that in A Feast for Crows she leaves Westeros and begins training to become a faceless man herself.