Arnold was a victim of slavery. His ears, the pride of a beastman, had been taken away eternally just because his master happened to be in a bad mood. He was able to rid of the magic lock crest and fled and had someone that knew about the cruel treatment of slaves kill his master. With the death of his master, Arnold officially became free. After he got his freedom, he became a travelling cook. Arnold was under Rarashik's tutelage after Judom forced her to. (Source: Konjiki no Word Master Wikia)