agent

determinant, expedient, factor, influence, ingredient, mechanism, tool

weapon

activator, animator, catalyst, driver, energizer, executor, generator, impetus, incentive, inspiration, instigation, instigator, launcher, mover, power, stimulus, trigger

antecedent, cause, occasion, reason

subagency, subagent

ambassador, diplomat, diplomatist, emissary, foreign minister, legate, plenipotentiary

alternate, backup, pinch hitter, relief, replacement, stand-in, sub, substitute, surrogate, understudy

informer, operative, spy

broker, distributor, manager

arbiter, arbitrator, comprador (or compradore), conciliator, go-between, intercessor, intermediary, interposer, liaison, mediator, middleman, peacemaker

mouthpiece, point man, point person, prophet, speaker, spokesman, spokesperson

ambassadress

attaché, chargé d'affaires, consul, deputy, diplomat, foreign minister, nuncio, procurator, proxy

apostle, evangelist, missionary

deputation, detachment, legation

courier, messenger

mouthpiece, spokesperson

courier

counterspy, double agent, sleeper

infiltrator, informer, stool pigeon

spymaster, superspy