Mion is the oldest classmate in Keiichi's class and is the twin sister of Shion Sonozaki. As the elder, she is the class president and everyone looks up to her as the leader. Her social skills are on par with Keiichi, whom she highly respects as a friend and rival (though it is later shown that she has somewhat of a crush on him). She acts like a tomboy, and calls herself ojisan (meaning old man), but has a hidden girlish side. She is next in line to be the head of the Sonozaki household, one of the Three Families which hold tremendous influence in Hinamizawa. She is often seen with a holstered airsoft gun.