collaborator

(noun) someone who assists in a plot. Synonyms: confederate, henchman, partner in crime.

(noun) someone who collaborates with an enemy occupying force. Synonyms: collaborationist, quisling.

(noun) an associate in an activity or endeavor or sphere of common interest. Synonyms: cooperator, partner, pardner. Examples: "The musician and the librettist were collaborators." "Sexual partners."