He is an ex-member of The Companions. He was asked to leave because his "unorthodox" methods were offensive to the members. It is unclear what these methods are or how they were offensive. He joined The Circle to become a Werewolf.
He initially disapproves of the Dragonborn, demonstrating attitude and a slight essence of rudeness, but after several successful assassinations, he begins to respect and even envy the hero.
If the Dragonborn is a werewolf, however, he will immediately know and react a bit more amiably. Regardless of whether they are a werewolf or not, Arnbjorn will use little nicknames such as lambshank, beef roast, tidbit, or hamhock. He says not to take offense from the nicknames, since it's hard for him to not to think of people as a snack, stereotypically as a werewolf would be expected to.
Arnbjorn is terribly wounded by Cicero part way through the Dark Brotherhood questline (found outside the Dawnstar Sanctuary). He is then later killed during the assault on the Dark Brotherhood Sanctuary, where he dies in Werewolf form while holding off Penitus Oculatus agents. (His death is scripted and he will still die even if the Penitus Oculatus are killed first.)
After dying in beast form at the hands of Penitus Oculatus Agents, his dead body will contain a wolf pelt. With the addition of Dawnguard, this remains a wolf pelt rather than becoming a Werewolf Pelt.
- He wears a unique short sleeved variation of the Shrouded Armor. If the Dragonborn steals his armor and equips it, it will have long sleeves.
- He never wears any sort of footwear, most likely due to his lycanthropy.
- He's the only character who will never give any useful advice to them when asked for one before a mission (apart from Cicero who doesn't have that dialogue option).
- Due to his use of two handed weapons he can sometimes be seen in the forge part of the sanctuary attacking nothing because of movements made by two handed attacks.
- Arnbjorn is a combination of Old Norse for Eagle and modern Scandinavian for Bear.
- When practicing on the dummies in the sanctuary, he may be seen using a dual one-handed power strike, even though he is using a two-handed battleaxe.
- The Elder Scrolls V: Skyrim (First appearance)