intercessor

n. person who intercedes, one who pleads or speaks on behalf of another; mediator

Intercession, in both Christianity and Islam, is a prayer to God on behalf of another person.
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Noun
1. a negotiator who acts as a link between parties
(synonym) mediator, go-between, intermediator, intermediary
(hypernym) negotiator, negotiant, treater
(hyponym) mediatrix

n. intercessor, mediator

(n.)
One who goes between, or intercedes; a mediator. (a) One who interposes between parties at variance, with a view to reconcile them. (b) One who pleads in behalf of another.
  

(n.)
A bishop, who, during a vacancy of the see, administers the bishopric till a successor is installed.