1.
ringleader; mastermind; instigator
The person who plans and leads a criminal act, conspiracy, or organized wrongdoing. The one behind the scheme.
事件の首謀者が逮捕された。
The mastermind of the incident was arrested.
首謀者はまだ特定されていない。
The ringleader has not yet been identified.
警察は詐欺グループの首謀者を追っている。
The police are pursuing the mastermind of the fraud ring.
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).