Ryoko Ninomiya is one of the supporting protagonists of light novel, manga, and anime series Instant Death, she's one of the Sage Candidates and one of Yogiri Takatou's companions. She is one of the people who realizes how minacious he really is due to the seal of Yogiri and her experience with him in the original world they originally came from.
In the old world, she is sent by the Institute to infiltrate Yogiri's class and watch him.