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
That kid is a tattletale who immediately tells the teacher.
Because of the snitch, everyone's secret got out.
Hiragana variant of the first part; less common but still seen.
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.