noun
telltale; tattler; snitch
Colloquial, often used by children or in casual contexts for someone who secretly reports others' misdeeds. The 魔 suffix adds a negative, almost demonic connotation.
See also: ちくり
あの子はすぐ先生に言いつけるチクリ魔だ。
That kid is a tattletale who immediately tells the teacher.
チクリ魔のせいで、みんなに秘密がばれた。
Because of the snitch, everyone's secret got out.
密告者 is a formal term for an informant or whistleblower, while チクリ魔 is a childish or slangy label for a petty tattler.
Compound of チクリ (mimetic for sneakily revealing secrets or tattling) and 魔 (demon, person with a certain trait). The exact origin of チクリ is uncertain, but it is sound-symbolic.