Hugo Wull is the clan chief of House Wull, one of the Northern Mountain Clans. Out of the clan chiefs he has the most men. Hugo is known as The Wull or Lord Wull. Due to the Wull sigil being three buckets he is also known as Big Buckets.
When Stannis Baratheon arrives in the North he intends to gather the Northmen to wrest control of the North from Roose Bolton, who treacherously murdered his King Robb Stark at the Red Wedding and was made Warden of the North by the Lannisters. Stannis originally intends to march on the Bolton seat, the Dreadfort, aided by Arnolf Karstark (who is secretly working with the Boltons to lure Stannis into a trap). However on the advice of Jon Snow Stannis instead rallies the Mountain Clans and marches on Deepwood Motte, the last Ironborn-held area in the North. Stannis captures Asha Greyjoy, who had taken Deepwood Motte, and restores it to the Glovers, who join him, and the Mormonts declare for him as well.
After Stannis' army leaves Deepwood Motte to march on Winterfell, they are waylaid by heavy snows. The Northmen are used to this weather, however much of Stannis' army is made up of Southron forces, who have difficulty coping. The Mountain Clans encourage them to keep marching, but Corliss Penny accuses Wull of being in league with the Boltons. Wull says that men always die in war and that this will be his last winter, he wants to die fighting, which many of the Northmen agree with. Stannis' army is finally forced to halt in an abandoned crofter's village three days from Winterfell, where they are joined by Arnolf Karstark.
Theon Greyjoy escapes Winterfell with "Arya Stark" (really Jeyne Poole, who was forced into the role so she could marry Ramsay Bolton and legitimize Bolton rule). They are found by another of the Northmen who have joined Stannis, Mors Umber, and brought to Stannis. The Northmen, including Hugo, want Theon dead due to him betraying his foster-brother Robb Stark and joining the Ironborn in attacking the North, and apparently killing his foster-brothers Bran and Rickon Stark. Stannis has the Karstarks arrested and agrees to meet Hugo Wull after meeting Asha Greyjoy.