Styr is a wildling and the Magnar of Thenn, the leader of one of the fiercest tribes of the Free Folk, and a lieutenant of Mance Rayder.
Styr is the Magnar of the Thenn, a fierce tribe of Free Folk known for their practice of ritual self-scarification and engaging in cannibalism of their fallen enemies. Styr is a very highly-skilled warrior and is capable of wielding a great axe with ease due to his size and strength.
At some point, Mance Rayder convinced Styr to join his army and acknowledge him as the King-Beyond-the-Wall. Styr became one of Mance's lieutenants, along chieftains such as Tormund Giantsbane and the Lord of Bones.
Styr leads a group of Thenns towards Castle Black and encounters Tormund and Ygritte after the warg in his group sees them through the eyes of his owl. Styr mocks them over the death of Orell and their failure to kill Jon Snow. Afterwards, he and his fellow Thenns prepare to eat a member of the Night's Watch whom they have recently killed.
Styr, Tormund, Ygritte and other wildlings attack a village killing all of its residents, save for one child. Styr orders this child to run to Castle Black to warn the Night's Watch about the wildlings. Before this, he informs the child that he is going to eat his dead parents.
Styr and Tormund attack the settlement of Mole's Town. They slaughter most of the inhabitants, which include members of the Night's Watch who were visiting the local brothel. News of this attack reaches Castle Black, which is their next target.
Just outside of Castle Black, Styr chastises Ygritte for not killing Jon Snow when she had the chance. Soon after, his warg has spotted Mance Rayder's signal and they attack Castle Black. During the battle, he kills numerous black brothers before spotting Jon Snow. In the midst of the fighting, Styr and Jon duel each other in the courtyard of Castle Black. Styr gets the upper hand by knocking away Longclaw from Jon and fighting hand to hand. He forces Jon's head into a blacksmith anvil, however, Jon eventually distracts him by spitting in his face and literally smashes Styr's head in with a hammer, killing him.
After the arrival of Stannis Baratheon and his army, Jon enters the chambers where Tormund is being held and mentions that the bodies of the fallen wildlings are going to be burned. Styr's body is burned along with the other wildlings.
Leader of the Thenn wildlings, Styr was a fierce warrior with a hunger for human flesh. He was killed by Jon Snow during the Battle of Castle Black.