Mion Sonozaki is a main character and the heir to the Sonozaki family, one of the three influential families that control the village of Hinamizawa. She serves as the class president and leader of the after-school games club, a position that reflects her natural authority and social confidence. Mion is the oldest student in her class and is often looked up to by her peers for her leadership and maturity.

Her background is marked by a complex family dynamic involving her twin sister, Shion. Due to the customs of the Sonozaki household, the title of Mion and the role of firstborn heir are reserved for the elder twin. In reality, Mion was born second, originally named Shion, while her sister was born first and originally named Mion. As children, the twins decided to swap identities so that both could attend family meetings, but this swap became permanent when the firstborn was branded with a demon tattoo. Mion received the tattoo instead of her sister, officially becoming the recognized heir. This secret identity swap is known only to the twins themselves and adds a layer of personal tragedy to their relationship, as Mion carries the weight of a responsibility that was never meant to be hers.

Mion's personality is a blend of boisterous tomboyishness and hidden vulnerability. She frequently refers to herself as oji-san, meaning old man or uncle, a playful self-characterization that underscores her fun-loving and competitive nature. She thrives on games and strategy, often orchestrating elaborate club activities with unusual penalties, and she is not above bending the rules or using clever tricks to win. Despite this mischievous side, she possesses a strong sense of responsibility, especially when acting as the Sonozaki heir, at which times she becomes formal, serious, and almost expressionless. She also has a softer, more girlish side that she rarely shows, most notably evident in her secret crush on Keiichi Maebara, which she expresses through teasing and rivalry.

Mion's motivations are rooted in a desire to protect her friends and fulfill her duties as heir. She genuinely cares for her club members and often acts as a stabilizing force within the group. Her role in the story is significant because she is the only main character who does not succumb to extreme paranoia or commit murder in the main arcs. She remains grounded and resolute even as the events around her spiral into violence and suspicion. This makes her a key anchor for the group, providing a sense of normalcy and resilience amid chaos.

Key relationships define much of her development. Her bond with her twin sister Shion is both affectionate and strained, marked by guilt, protectiveness, and the hidden secret of their swapped identities. With Keiichi Maebara, she shares a competitive friendship that gradually reveals her romantic feelings, though she struggles to express them directly. Her interactions with the other club members, Rena Ryuuguu, Satoko Houjou, and Rika Furude, are characterized by camaraderie and playful rivalry, and she often takes on a big sister-like role within the group.

Mion's notable abilities include a high level of martial arts proficiency, sufficient to defeat trained opponents in hand-to-hand combat. She is also an expert strategist, particularly in games that involve psychological manipulation and cunning. While she is sometimes depicted carrying an airsoft gun, she rarely uses it, and her true strengths lie in her physical skills and tactical mind. Throughout the story, her character shows growth as she confronts the darker truths of her family's history and the village's secrets, learning to balance her duty as heir with her personal desires. She remains a loyal and steadfast friend, and her hidden girlish side occasionally emerges when she allows herself to be vulnerable.