One who relays confidential information to someone, especially to the police; an informer.
A native speaker who acts as a linguistic reference for a language being studied. The informant demonstrates native pronunciation, provides grammaticality judgments regarding linguistic well-formedness, and may also explain cultural references and other important contextual information.
One who gossips, often for the sake of attention.
One who tattles (reports others' wrongdoings), often a child seeking attention.
telltale; giveaway
To act as a tattletale; to tell on; to give away, reveal, or expose.