noun
Refers to someone who secretly reports others' activities to authorities or an enemy; often carries a negative connotation similar to 'rat' or 'snitch'.
彼は警察の密告者だった。
He was a police informer.
密告者が誰か分からない。
I don't know who the snitch is.
情報提供者 is a more neutral term for 'informant', without the negative nuance of betrayal.
Compound of 密告 (secret report, betrayal) and 者 (person).