Mion is the oldest kid in Keiichi's class. As the elder, she is the class president and everyone looks up to her as the leader. She acts like a perverted tomboy and calls herself old man, but has a hidden girlish side. Mion is next in line to be the head of the Sonozaki household, one of the Three Families which holds tremendous influence in Hinamizawa. Often when playing games with her fellow club members she is known for her ruthlessness, often coming up with the most embarrassing penalties for the losers. Mion also serves as the leader of the main characters' gaming club.

Spoiler

Mion is more stable than the other club members, and as such has never herself succumbed to Hinamizawa Syndrome