agent
connection
deputy
entrepreneur
front
front man
go-between
interagent
interceder
intercessor
intermediary
intermediate
intermediate agent
intermedium
internuncio
intervener
interventionist
interventor
mediator
medium
middleman
mouthpiece
negotiant
negotiator
negotiatress
negotiatrix
ombudsman
spokesman
spokeswoman
treater
conciliator
diplomat
harmoniser
harmonizer
interpreter
make-peace
marriage broker
matcher
matchmaker
mediatrix
moderator
pacifier
peacemaker
reconciler
second hand
translator