Mion Sonozaki is one of the central characters in the story and the heir to the Sonozaki family, one of the three influential families that effectively govern the village of Hinamizawa. She is the twin sister of Shion Sonozaki. A crucial and often overlooked detail of her background is that Mion was actually born second. The name Mion, along with the position of family head, is traditionally reserved for the firstborn twin. Her sister was originally named Mion, but the twins occasionally swapped identities to allow both to attend family meetings. On the day the firstborn was to receive the ceremonial demon tattoo, they had swapped places, resulting in Mion permanently receiving the tattoo and the name Mion, while her sister took the name Shion. This secret is known only to the twins themselves. As part of her training to become the next family head, she lives with her grandmother, Oryou Sonozaki, and has been trained in various disciplines including martial arts, cooking, and the operation of vehicles and communication equipment.

Mion possesses a multifaceted personality that is deeply shaped by her dual roles as a schoolgirl and a yakuza heiress. In her daily life at school, she is energetic, cheerful, and a natural mood-maker, serving as the class president and the self-appointed leader of the after-school club. She has a boisterous and mischievous side, often using an old-man-like tone and calling herself oji-san. She loves games and competition, frequently organizing strategy games for her friends and employing cunning, rule-bending tactics to win. Despite her tomboyish exterior, she has a deeply hidden girlish side and is shy about expressing her romantic feelings for Keiichi Maebara. However, when she acts in her capacity as the Sonozaki heir, her demeanor shifts completely. She becomes formal, expressionless, and can be ruthlessly cold, a contrast that highlights the weight of her family responsibilities. Deep down, she carries guilt regarding her twin sister, feeling that Shion has been treated unfairly as the disfavored child, which sometimes prevents her from asserting herself in their relationship. A notable trait is that, despite the violent events that plague her friends, Mion is the only main character in the original question arcs who never commits murder or succumbs to the extreme paranoia that drives the others to violence, serving as a grounding and stabilizing presence.

Her primary motivation is the well-being and protection of her friends, whom she cherishes dearly and would risk her life to defend. She is also driven by her duty as the heir to the Sonozaki family, a role that requires her to balance personal feelings with the cold pragmatism expected of a future leader. Her competitive drive is a significant motivator, fueling her love for the club's games and her complex dynamic with Keiichi, whom she sees as both a friend and a rival.

In the story, Mion serves as a key leader figure and a crucial link between the ordinary world of the school club and the dark, secretive history of Hinamizawa. Her position within the Sonozaki family makes her a central figure in the village's power structure, and her knowledge of its secrets is vast. Her role often involves managing the tension within the group and acting as a stabilizing force during periods of crisis. Her mistakes in communication, particularly her failure to be honest about her feelings or to share critical information in a timely manner, have been known to inadvertently trigger tragic events.

Mion's key relationships are central to her character. Her friendship with Keiichi Maebara is one of mutual respect, rivalry, and unspoken romantic affection. She also has a strong friendship with Rena Ryuuguu, whom she considers both a best friend and a rival for Keiichi's attention. Her bond with her twin sister, Shion, is the most complex and emotionally fraught. While they love each other, their relationship is strained by the secret of their swapped identities, Mion's guilt, and Shion's feelings of being overshadowed. She also shares a deep, protective bond with the other club members, Rika Furude and Satoko Houjou, and serves as their de facto big sister figure.

Mion undergoes significant development throughout the narrative. Initially presented as a purely fun-loving and confident leader, her character deepens as the story explores the heavy burdens of her family legacy. She is forced to confront the moral compromises required by her position and learns to reconcile her tomboyish nature with her more vulnerable, feminine side. A key moment of growth occurs when she accepts a stuffed doll from Keiichi, an act that subverts a previous tragic future and demonstrates her ability to be honest about her hidden desires. She ultimately learns to assert her own will and work with her friends to overcome the fate that has been set for them.

Notably, Mion is highly skilled in martial arts, proficient enough to fight evenly with the trained mercenary Okonagi. She is a brilliant strategist and a master of tabletop games, excelling at deception and dirty tricks. While she often carries a model gun, she rarely uses it, and her true abilities lie in her tactical mind, her leadership, and her remarkable physical prowess. She also possesses a surprising talent for small-unit tactics, having effectively outmaneuvered professional operatives during the events of the Dam War.