Westonji Ruiou, a member of the Special Crimes Unit. Five years ago, she lost her older sister, Mayo. Mayo was also part of the Special Crimes Unit and an excellent detective. However, while undercover investigating the notorious criminal organization Mantis, she tragically died during the course of her investigation due to an accident that remains shrouded in mystery. Driven by vengeance and fueled by her unwavering determination, Ruiou continues to pursue Mantis, relentlessly seeking justice for her beloved sister.

［姉の仇、マンティスを追い続けて…］西園寺莉緒、特殊犯罪課に所属。5年前に姉・真緒を亡くしている。真緒も特殊犯罪課に所属しており、優秀な捜査官だった。しかし、地下犯罪組織マンティスの潜入捜査を行っていた際、捜査中の事故により死亡。詳細は明らかにされていない。