Jon Umber is the eldest son and heir of Lord Jon Umber of the Last Hearth. He is known as the Smalljon to distinguish him from his father, the Greatjon. Though smaller then his father, who is nearly seven foot, he is nearly as tall as them and could have become taller still. Like his father he is strongly built.
When Robb Stark calls the Stark banners the Umbers are among the vassals who come to Winterfell, the Smalljon accompanying his father. Smalljon Umber is one of the young nobles who make up the personal guard of Robb, fighting with them during the Battle of the Whispering Wood. At Riverrun the assembled nobility of the North and Riverlands debates how to act in response to the execution of Robb's father, Eddard Stark, by Joffrey Baratheon. The Smalljon's father proclaims Robb King in the North, and is followed by the other nobles in doing so.
When Robb attacks the Westerlands he is accompanied by the Smalljon, who scales the walls of the Crag with Black Walder Frey . However Robb takes the virginity of Jeyne Westerling, whose father is Lord of the Crag, and marries her to preserve her honour, breaking the marriage pact he made with Lord Walder Frey to marry a Frey. Due to Stannis Baratheon being defeated at the Battle of the Blackwater the Freys think the Lannisters will win the war and are trying to find an excuse to break with Robb.
The Freys tell Robb they will rejoin him if his uncle Edmure Tully, Lord of Riverrun, marries Roslin Frey, a daughter of Walder Frey. Smalljon Umber goes with Robb to the wedding, guarding him. However the wedding is a trap, becoming known as the Red Wedding. Aided by Robb's treacherous bannerman Roose Bolton the Freys destroy most of Robb's army and his mother, Roose personally murdering Robb. Smalljon Umber tries to protect his King when he is wounded by crossbow bolts, throwing a table over Robb to protect him from further bolts. However Jon is wounded by a crossbowman while running for his swordbelt, falling to his knees. He is then beheaded by a Bolton man with two swings of an axe.