Mion is a tomboy and the leader of the after-school games club she has with her friends. She always reminds them of the rules if they try to slack during games or get out of them, often leading to ruthless punishments conjured by her such as being written on or wearing an embarrassing outfit in public. She's a confident, energetic girl, and the heir to the Sonozaki family estate. When dealing with family matters she's very serious and often expressionless, staying on the topic at hand. Deep down Mion is actually quite girly, but is pretty embarrassed about showing this side of her, so she keeps it hidden. Mion also has a crush on Keiichi, though she's very hesitant to open up about it. Overall, Mion is a wonderful leader and cares deeply for her friends and sister, willing to do anything to protect them at all costs. She often refers to herself as "Uncle Mion" or "old fart."

Mion's voice is mid/mid-high. She doesn't have any special way of speaking but does sound a bit gruff.