In Game of Thrones season 6, episode 3, 'Oathbreaker', we see that Bran and the three eyed raven are mentally traveling back to a time where Eddard was a young lord.
Now, the memory that Bran and the three eyed raven travel to, is when Bran's father and a group of men take on the Kingsguard Knights to get to Eddard's sister who is locked away in a tower. The significance of this, is that at the end of the fight, Eddard isn't the one who struck the significant blow to beat Arthur Dayne, but apparently Eddard has told the story to his sons saying that he was the one who killed Arthur Dayne, which makes Bran think that the rest of Eddard's stories might also be false.
I might've missed the part where that man was introduced or explained due to the fact that Game Of Thrones has so much going on all the time and me getting up and down from my chair. But who is the man who helped Eddard defeat Arthur Dayne, and what is his significance in this show?