He is a wildling Giant who resided at Hardhome before being saved by Jon Snow and taken south of the Wall. He is more commonly referred to by the other wildlings as "Wun Wun".
Wun Wun is a Giant, possibly the only one to reside in Hardhome and possibly the last giant to ever exist.
Wun Wun is present when Jon Snow and Tormund Giantsbane battle against the White Walkers. Wun Wun appears willing to support Tormund casting his vote simply by stating Tormund's name. Later, when the Night's Watch is gathering the dragonglass that Jon shared with the wildlings, as it can kill the White Walkers, Wun Wun is holding a dragonglass arrowhead, and Eddison Tollett approaches to tell the giant to give it up, though he is too afraid to do so because of his intimidating size. Wun Wun gets annoyed by his inaction and asks "The fuck you looking at?" in the giants' rudimentary form of the Old Tongue, so Edd goes away quietly, without collecting the arrowhead, Wun Wun may still posses this piece of dragonglass. When the Wights attack the village, Wun Wun single-handedly defeats about a dozen of them by ripping them apart, stomping on them, and defending himself with a large burning log. He then retreats into the sea with Jon and the remaining defenders. Being too large for the rowboats, he simply wades out into the bay for a long distance (not drowning due to his great height) to reach the larger ships waiting off shore. He is one of the few wildlings to escape the Massacre at Hardhome, and the fleet travels back to the Wall.
Alongside the other wildlings, he passes the Wall and leaves Castle Black to settle in the Gift. Many brothers of the Night's Watch are intimidated by him, having never seen a giant before.
Wun Wun and Tormund lead the wildlings in an attack on Castle Black, this time to save Jon's body and his few loyalists from the mutineers. Wun Wun breaks down the gates and intimidates the brothers into quickly surrendering, only beating one to death after he foolishly (and futilely) shoots the giant in the back with a crossbow. After Tormund orders the brothers to stand down, Wun Wun witnesses Ser Alliser and Olly being taken into custody for their role in the mutiny and the murder of Jon Snow.
When Jon, Sansa and Tormund attempt to recruit the wildlings, Wun Wun is present and is the first to cast his vote by saying 'Snow'.
Wun Wun is on the front lines of Jon's army. During the battle, after the cavalry enters and Ramsay's archers stop, Davos leads Wun Wun and his archers in a charge, as they cannot fire from their current position without hitting their own men. Wun Wun fights until they are surrounded by Smalljon Umber and the Bolton army. Wun Wun pulls a soldier out and rips him in half, but is unable to do major damage, being pierced by their spears. However, Petyr Baelish arrives with the knights of the Vale, his aid having been finally accepted by Sansa.
Ramsay's forces are decimated by the Vale cavalry, forcing him to flee inside Winterfell, believing that the Stark/Wildling/Vale army doesn't have the necessary equipment for a siege. Wun Wun proves him wrong, breaching the doors, but is riddled with arrows and crossbow bolts by the few remaining Bolton archers. Wildlings breach Winterfell and clear the remaining Boltons. Wun Wun falls to his knees, bloodied and injured from the battle, looks towards Jon and is shot in the eye by Ramsay Bolton. In response, Jon rushes Ramsay and beats him mercilessly, has him dragged away and later, Sansa has him fed to his own starving hounds, finally killing the Bastard of Bolton.
The last of the giants, Wun wun helped Jon Snow's group escape the White Walker ambush at Hardhome. He supported Jon at several critical junctures, and died helping Jon retake Winterfell in the Battle of the Bastards.