"Mion Sonozaki" (園崎 魅音, Sonozaki Mion) is a character in Higurashi no Naku Koro ni appearing exclusively in Yoigoshi-hen. She claims to be Mion Sonozaki, a former resident of the now devastated Hinamizawa. "Mion" is skilled with both swords and guns though she rarely strikes to kill. She is eventually revealed to be both Mion Sonozaki and Shion Sonozaki. While the real Mion did die during the Hinamizawa school explosion, Shion lived as she was in Okinomiya at the time. Akane Sonozaki assumed the role of head of the Sonozaki family after Mion and Oryou Sonozaki's deaths before being killed by an assassin sent by Mifune. Mion's spirit appeared in a dream instructing Shion to return to Hinamizawa. After arriving in Hinamizawa, Mion's spirit assumes control of Shion's body until the end of Yoigoshi-hen.
"Mion" returns to Hinamizawa in order to reclaim the Sonozaki family headship by taking the handbell that traditionally symbolizes said headship. The bell's location is a closely held secret that only Oryou knew. "Mion" uses a scroll located in the Furude shrine and her sword to find the bell but is repeatedly thwarted by members of the Sonozaki yakuza under the command of Mifune. After arriving at the Sonozaki Estate, "Mion" kills Mifune and retrieves the bell. Once she has the bell, Mion's spirit leaves Shion's body and "Mion" becomes Shion once again.
Full-body sprite of "Mion" in Higurashi no Naku Koro ni Kizuna