Cobb's companion. It's his job to do background checks on the individuals who's dreams they invade, he plays a key role in bringing the others out of the dream.
Arthur is Cobb's right hand man in the extraction jobs they pull. He usually works closely with him to set diversions and convince their "mark" (the target of extraction, i.e. the dreamer) of whatever they need him to. He is also typically responsible for getting people awake by synchronizing "kicks" and also provide intelligence and backstory on their next job. It is unknown how long Arthur has worked along side Cobb but clearly long enough to know about his dead wife, Mal, and his assumedly false charges against her murder. However, even though Arthur presses for more information on why Cobb's projection of Mal continuously causes problems for them inside of another person's sub-conscience, Cobb remains distant in telling Arthur and he begins to suspect that he doesn't quite know everything.