collaborator

Noun

• an associate in an activity or endeavor or sphere of common interest
  • the musician and the librettist were collaborators
    
  • sexual partners
    

• someone who collaborates with an enemy occupying force

• someone who assists in a plot

Related words

• Synonyms: 
  cooperator , partner , pardner , collaborationist , quisling , confederate , henchman , partner in crime