Balon Greyjoy: The King of the Iron Islands
Robb Stark left an opening, and Balon Greyjoy exploited it. It's pretty much as simple as that, and that's how the recidivist rebel king of the seafaring Ironborn conquered the North and became a major player after years on the sidelines. Scratch the surface of the victory and you'll see how shallow it runs, though: His family is more fractious than even the Starks or Lannisters at this point, with both him and his daughter Yara leaving his estranged son Theon to take the fall when the Northmen reconquer Winterfell. Again, it's unclear what happened there, or what became of Theon when it happened, so Balon's plans for the future had best be able to survive losing both the capital of the North and the son who briefly ruled it.