noun

a person who works jointly on an activity or project; an associate.

his collaborator on the book

co-worker,fellow worker,associate,colleague,partner,co-partner,confederate,ally,teammate,assistant,helper,oppo,quisling,fraternizer,collaborationist,colluder,(enemy) sympathizer,conspirator,traitor,fifth columnist,renegade,betrayer,turncoat,defector,informer,double agent