Smalljon was the son of Greatjon Umber, and was the Lord of Last Hearth.
Smalljon Umber is the eldest son and heir of Jon Umber. After his father's death, he becomes the new Lord of Last Hearth and head of House Umber. Although he dislikes House Bolton, he sides with Ramsay Bolton to protect the North against the wildlings.
Smalljon Umber travels to Winterfell to treat with Ramsay Bolton, requesting aid to fight the wildlings that have been let into the North by Jon Snow. Ramsay questions his motives, since the Umbers are famously loyal to House Stark, but Smalljon notes that House Karstark has Stark blood, yet Harald Karstark is Ramsay's new lieutenant. Smalljon calls Ramsay's father Roose a cunt, and when Ramsay attempts to praise Roose, he interrupts him, again calling him a cunt, and correctly accuses Ramsay of murdering him, despite Ramsay's persistent claim that Roose was poisoned by his enemies.
Smalljon reveals that Jon Snow has let a wildling army south of the Wall, which is bad news for the Umbers as Last Hearth is further north than any other castle, and it is always the Umbers who are forced to fight the wildlings first, something Smalljon enjoys. This time, however, there are too many wildlings, and Smalljon requests Ramsay's help. Smalljon refuses to swear fealty to Ramsay, or even kneel. Instead, he presents Ramsay with the captives Osha and Rickon Stark. When Ramsay asks Smalljon how he knows the boy is really Rickon Stark, he provides the severed head of Rickon's direwolf, Shaggydog, as proof.
Smalljon, Ramsay, and Harald attend a parley with Jon Snow, Sansa Stark, and Davos Seaworth, offering terms for surrender. When Sansa asks for proof that Ramsay has Rickon, Smalljon tosses Shaggydog's head onto the ground. He and his soldiers take part in the Second Battle of Winterfell, where he watches as Ramsay leads a leashed Rickon onto the battlefield. Smalljon leads the Bolton spearmen who form a wall around the Stark army, rallying them with a cry of "Who owns the North? We do!" When the spearmen surround the Starks, Smalljon personally enters the frey and attempts to fight Jon Snow. Instead, he is attacked by Tormund, who he beats into submission. He becomes trapped with the Stark allies when the spearmen begin to crush them, but continues to beat Tormund with his head. He is momentarily distracted by the arrival of the Knights of the Vale, giving Tormund enough time to bite his neck and then stab him to death.
An enormous man with violent inclinations, smalljon bore no love for the Boltons. Nevertheless, he sided with Ramsay to secure support against the wildlings, the longtime enemy of the North. In return for Ramsay's backing, Smalljon turned over Rickon Stark and Osha. Smalljon fought for House Bolton iun the Battle of the Bastards and was killed by Tormund