accomplice; partner in crime
Refers to a person who participates in a crime with another. Often used in legal or criminal contexts.
He was arrested as an accomplice in the robbery.
It seems the accomplice is still on the run.
共犯者 specifically means an accomplice in a crime, while 相棒 can mean the same thing informally but also just a regular work or activity partner.
Compound of 共犯 (complicity) + 者 (person).