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