communicator - a person who communicates with others

betrayer, blabber, informer, squealer, rat - one who reveals confidential information in return for money

leaker - a surreptitious informant; "the president wanted to know who the leakers were"

passive source - an informant who is not assigned to obtain specific intelligence but who routinely passes on whatever information he or she has

whistle blower, whistle-blower, whistleblower - an informant who exposes wrongdoing within an organization in the hope of stopping it; "the law gives little protection to whistleblowers who feel the public has a right to know what is going on"; "the whistleblower was fired for exposing the conditions in mental hospitals"

beholder, observer, perceiver - a person who becomes aware (of things or events) through the senses

attester, attestant - someone who affirms or vouches for the correctness or truth or genuineness of something

speaker, talker, verbaliser, verbalizer, utterer - someone who expresses in language; someone who talks (especially someone who delivers a public speech or someone especially garrulous); "the speaker at commencement"; "an utterer of useful maxims"

deponent, testifier - a person who testifies or gives a deposition