In Yoigoshi-hen, Otobe Akira finds himself in the company of a woman with a sword who claims to be Sonozaki Mion, a former resident of the now devastated village of Hinamizawa. She initially mistakes him for Hōjō Satoshi, decides it's fate and allows him to accompany her.
She is blunt, highly perceptive, and not afraid to speak her mind. She is the one to realize the truth behind the fate of Kurosawa Takumi and promptly addresses it and has no problem calling Akira on the holes in his story, too. When she fights she does so with sword and gun but generally does not strike to kill, merely to disable her opponent. She very much enjoys sake, and when drinking seems to become a cheerful and slightly silly person.
According to Arakawa Ryūnosuke, the woman can't be Sonozaki Mion because Mion was one of the victims of the explosion at the school. As the story progresses she hears and sees things that can only be ghosts or memories of Mion's childhood friends, and the knowledge she has of the Sonozaki family, the village, and the local legends make it virtually impossible that she is anyone else.
The woman who claims to be Sonozaki Mion is, in fact, Sonozaki Mion and Sonozaki Shion. The real Mion did die in the explosion at the school, and after the Great Hinamizawa Disaster took her grandmother as well, Shion took on the name and identity of her twin sister, who was the heir to Sonozaki group. Because she was so young her mother stepped in for her and was able to hold the group together until she was shot and killed by command of Mifune. Mion's spirit had stuck close to her twin after death and appears to her in a dream instructing her to go to Hinamizawa. She is pursued and run off the road. At this point, Mion apparently takes control of her sister's body. Shion does not come back to herself until the end of the story after Mion saves her life and accomplishes their goal.