1.
ringleader; mastermind; instigator
The person who plans and leads a criminal act, conspiracy, or organized wrongdoing. The one behind the scheme.
Composed of 首謀 (plotting, scheming) + 者 (person). Almost always carries a negative connotation — the person behind illegal or harmful activities. Common collocations: 首謀者を逮捕する (to arrest the ringleader), 首謀者を特定する (to identify the mastermind). Compare with 黒幕 (puppet master — someone pulling strings from behind the scenes) and 主犯 (principal offender — the legal term).