Synonyms for Co-worker:

n.

counterpart, associate, relief, opposite number, subordinate, junior, superior, workmate. ally (noun)
associate, partner, friend, colleague.
associate (noun)
comrade, crony, playmate, fellow, ally, mate, pal, companion, chum, buddy.
buddy (noun)
intimate, confidant.
co-worker (noun)
colleague, workfellow, fellow worker.
collaborator (noun)
teammate.
colleague (noun)
workmate.
companion (noun)
cousin, counterpart.
friend (noun)
buddy, classmate, friend, mate, Familiar, comrade, acquaintance, cousin, schoolmate, roommate, crony, company, playmate, companion, amigo, chum, fellow, confidant, teammate, intimate, pal, partner, sister, brother, colleague, ally.
mate (noun)
classmate, schoolmate, acquaintance, Familiar, roommate.
person (noun)
colleague, workfellow, fellow worker.
sidekick (noun)
amigo.

Other synonyms:

counterpart, workmate, opposite number. subordinate. superior. junior. relief. comrade
associate.
Other relevant words:
counterpart, workmate, workfellow, superior, relief, subordinate, junior, associate, fellow worker.