What is another word for "mediator"?

Roget's Thesaurus (1911) lists these synonyms for "mediator": abettor, accessory, accomplice, adherent, adjunct, adjutant, adjuvant, advocate, after, ally, alter, angel, arbitrate, arbitration, arbitrator, assistant, associate, auxiliary, backer, bargain, candle, champion, clerk, coadjutor, coadjutrix, collaborator, colleague, complice, compromise, confederate, confidant, confrere, consignee, cooperator, court, criminal, diplomacy, diplomatics, diplomatist, dupe.