Brandon Stark

Heir to Winterfell, hot-tempered and headlong where Ned was careful, Brandon rode to King's Landing after his sister vanished and demanded that Rhaegar come out and die. It was exactly the wrong thing to do to a king already lost to his own fire.

Aerys arrested him, summoned his father to answer for it, and killed them both in the throne room by a method designed to be watched. Brandon's death made Ned the Lord of Winterfell, gave him Catelyn Tully as a bride, and put the North irrevocably into the war. Almost everything in Robert's life turns on that afternoon.