The police arrested the thief but his accomplice escaped.

Friend : a person with whom one has a bond of mutual affection, typically one exclusive of sexual or family relations.
Colleague : a person with whom one works in a profession or business.
Companion : a person or animal with whom one spends a lot of time or with whom one travels.
Partner : either of a pair of people engaged together in the same activity.
Accomplice : a person who helps another commit a crime.

Synonym of Accomplice is Partner