Leathers is one of the 63 wildlings that Jon Snow takes up to Castle Black from Molestown. He and Jax are the first 2 wildlings of the 63 to join the watch. when he goes north of the Wall to take his vows with 5 other recruits, Tom Barleycorn finds 9 wildlings in the weirwood grove, including the giant Wun Weg Wun Dar Wun. Leathers helps Jon Snow calm them by talking to the giant in the Old Tongue.
Jon Snow later appointed Leathers master-at-arms at Castle Black over the objection of Bowen Marsh. Leathers serves as a translator for the giant and Jon Snow. He has been teaching Jon some of the Old Tongue so Jon can talk to the giants.
The recruits of the Night's Watch are terrified of their new master-of-arms but Jon thinks that is a good thing as they will need to familiarize themselves with that emotion and overcome it when the Others come.
Leather swore his Night's Watch vows in the same grove of weirwoods of the Haunted Forest where Jon Snow and Samwell Tarly swore theirs. With him were fellow wildling Jax and four other recruits that followed the Old Gods. He was assigned as a ranger.