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.
密告者 focuses on secretly reporting someone's wrongdoing to authorities, while 裏切り者 is broader and doesn't require informing to officials.
Compound of 密告 (secret report, betrayal) and 者 (person).