agent
/ˈeɪdʒənt/
noun
A person who acts on behalf of another person or group.
in the event of illness, a durable power of attorney enabled her nephew to act as her agent
representative,negotiator,business manager,emissary,envoy,factor,go-between,proxy,surrogate,trustee,liaison,broker,delegate,spokesperson,spokesman,spokeswoman,frontman,mouthpiece