Arnbjorn is a Nord assassin, werewolf, former Companion, husband to Astrid, and member of the Dark Brotherhood. He says Astrid doesn’t like to admit he’s her husband as she doesn’t want anyone to think she plays favorites. When asked about himself he claims all people need to know is he’s a werewolf, likes killing people, loves Astrid, hates annoying people, and the color blue gives him a headache. Although hostile to the player he is slightly less so if they are a werewolf but says his dislike is nothing personal, it’s just how he is. Arnbjorn will call the player names like ‘tidbit’, ‘ham shank, and morsel saying "I’m a werewolf. Hard not to think of you as a snack."
When Cicero fled the sanctuary after attacking Astrid for speaking ill of the Night Mother Arnbjorn chased after him and was nearly slain by Cicero but managed to grievously wound Cicero by hitting an artery which caused him to flee into the Dawnstar sanctuary where Arnbjorn couldn’t follow.