An older knight, Ser Alliser is a ranger of the Night’s Watch, and one of the Watch’s senior officers. At Castle Black, he serves as the master-at-arms, tasked with training new recruits to serve in the Watch. However, Alliser is cruel taskmaster, setting the recruits against one another callously. He fought for the Targaryens during the Sack of King’s Landing, and did so bravely, but was captured; Lord Tywin gave him the choice of execution or taking the black. He is described as a slender man.
On the show, Ser Alliser’s physical appearance is somewhat different from what is described in the novels.
Thorne’s harsh methods prove demoralizing for Jon Snow and the other recruits, until Tyrion convinces Jon to make friends with the other young men and help them learn. Their growing solidarity is proven when Samwell Tarly arrives and Thorne attempts to get the boys to hurt them. Angered, Thorne later tells Sam and Jon about the horrors of winter beyond the Wall, and how much you must be able to depend on the other men of the Watch. When it comes time to name recruits to their places in the Watch, it appears Thorne had a hand in having Jon named a steward instead of a ranger, as he hoped to be. Matters grow worse when Thorne tells Jon that his father is a traitor, leading to Jon attacking him and then being put in confinement in his room. After the attack by the wights on Mormont and others, Mormont dispatches Thorne to Eastwatch to take ship to King’s Landing and tell the court about what had happened.