The Greatjon is the Lord of Last Hearth and a formidable and proud bannerman of House Stark. He is fighting for Robb Stark in the War of the Five Kings. He is not present in the second season, as he is leading northern forces fighting in the Riverlands, nor in the third and fourth seasons. His role as Robb's chief northern advisor is taken up by Lord Roose Bolton.
Jon Umber is the Lord of Last Hearth and the head of House Umber, a loyal vassal family to House Stark of Winterfell. He is a large, proud, and formidable warrior.
Robb Stark sends ravens to the bannermen of House Stark instructing them to assemble for a march south to King's Landing to free Eddard Stark. The Greatjon leads a contingent of Umber troops to join the Stark army assembling at Winterfell. He is furious when Robb appoints Galbart Glover to lead the vanguard of the army during a feast at Winterfell. The Greatjon demands the honor for himself and refuses to follow a Glover into battle, threatening to take his troops home. Robb challenges him by saying that he will arrest him as a traitor on his return from the South. The Greatjon draws his knife and Robb's direwolf Grey Wind barrels down the table and knocks him off his feet. Grey Wind bites off two of the Greatjon's fingers. Robb mercifully offers that The Greatjon drew his knife in order to cut his meat for him and the Greatjon laughs off the incident calling Robb's meat tough.
Catelyn Stark joins the army at Moat Cailin. The Greatjon vows that they will smash the Lannisters and march on to King's Landing to rescue Lord Eddard. He objects to Robb releasing a Lannister scout from custody, but Robb exercises his authority and the Greatjon reluctantly accepts his decision. The scout had overestimated the size of Robb's fWhile King Robb Stark invades the Westerlands, Greatjon Umber is tasked with liberating the Riverlands from Lannister occupation. Lord Umber expels the Lannisters from Raventree Hall, then restoring the ancient castle of House Blackwood, and then wins another victory at Stone Hedge, which is then reclaimed by its rightful lord, Jonos of House Brackenorces.
After hearing of the execution of Lord Eddard, the Greatjon declares that he will never accept any king of House Baratheon as his ruler again, and leads the bannermen of House Stark as well as the Riverlords in proclaiming Robb the King in the North.
While King Robb Stark invades the Westerlands, Greatjon Umber is tasked with liberating the Riverlands from Lannister occupation. Lord Umber expels the Lannisters from Raventree Hall, then restoring the ancient castle of House Blackwood, and then wins another victory at Stone Hedge, which is then reclaimed by its rightful lord, Jonos of House Bracken.
The Greatjon is not present at the Twins during the events of the Red Wedding, making him one of the few bannermen of House Stark that remains alive and free.
When the Greatjon's son, Smalljon Umber, arrives at Winterfell to hand Rickon Stark over to Ramsay Bolton, he reveals that the Greatjon has died, and Ramsay accordingly addresses the Smalljon as "Lord Umber". He tells Ramsay he might have killed his father himself if he had not "died on his own", implying that his relationship with his father was not a pleasant one. Smalljon then betrays his fathers beliefs and House Umber's previous loyalty to House Stark by offering Rickon and Osha as a gift to House Bolton, to be held as prisoners and used as leverage against the Starks.
Jon Umber, of House Umber, is a principal bannerman of House Stark and a veteran of the battlefield.