Refers to someone who secretly reports others' activities to authorities or an enemy; often carries a negative connotation similar to 'rat' or 'snitch'.
He was a police informer.
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).