Also known as: 世凪
The main female character of Hakuchuumu No Aojashin Case-0. She is a gorgeous girl with White Hair and striking Red Eyes. Yonagi rarely speaks a word, shows little emotion, and acts in a very childish manner, appearing to have some kind of impediment. However, Kaito (Hakuchuumu No Aojashin) senses that there is more to her than meets the eye.
Later in the story, as Kaito is able to recollect his memories, he remembers that he had been living together with Yonagi in the bottom side since childhood. She is shown to have been an exceptionally bright, lively, and imaginative girl, the complete opposite of her current self. It is revealed that Yonagi is afflicted with an extreme case of Alzheimer's, which will deteriorate her behavior and even cause her to forget her entire identity.