Mion Sonozaki is a central figure in the story, serving as the outgoing and confident leader of the after-school club and the heir to the influential Sonozaki family in the village of Hinamizawa. She is the twin sister of Shion Sonozaki and the oldest student in her class, a position that naturally places her as class president and a respected figure among her peers. Her background is marked by a secret that defines her identity: she was born as the second twin, originally named Shion, while her sister was born first and named Mion. Because the name Mion and the role of family head were reserved for the firstborn, the twins occasionally swapped places so both could attend important family meetings. On the day the firstborn was to receive a demon tattoo as a mark of heirship, they had swapped, and Mion was permanently branded with the tattoo and permanently adopted the name Mion, a secret known only to the twins. This early act of unfairness and the subsequent burden of carrying a role not originally meant for her shaped much of her later sense of responsibility and protectiveness over her sister.

Personality-wise, Mion is energetic, cheerful, and highly sociable, with a natural charisma that draws others to her. She often adopts a tomboyish demeanor, playfully referring to herself as oji-san, or an old man, and she delights in competition and games. She is the organizer of the club's activities, creating elaborate games with creative rules and embarrassing penalties for losers, and she is adept at bending rules and using clever tactics to win. Despite this outward bravado, she possesses a hidden girlish side that she does not often show, particularly evident in her feelings for Keiichi Maebara, whom she respects as a friend and rival and later develops a crush on. She can also switch to a very serious and formal demeanor when acting as the heir of the Sonozaki family, a contrast that highlights her ability to separate her personal self from her public duties. Her motivations are driven by a strong sense of loyalty to her friends and her family, and she works to maintain peace and stability in her community, often acting as a grounding presence amid the chaos that unfolds.

In the story, Mion plays a role as both a protagonist and a pillar of the group. She is the leader who organizes club activities and mediates conflicts, and she is deeply involved in the village's hidden history and the dark secrets of the Sonozaki family, particularly during arcs that focus on the Cotton Drifting festival and the family's influence. A notable aspect of her role is that she is the only main character who does not fall into paranoia or commit murder in the core question arcs, maintaining her sanity even when others around her spiral into violence. This relative stability makes her an anchor for the narrative, though her family's legacy and the weight of her position still place her in the center of the tragedy.

Her key relationships include her twin sister Shion, whose contrasting personality and emotional volatility create a complex and often tragic dynamic between them. Mion loves her sister deeply but is also forced into harsh decisions when acting as the family heir, such as demanding Shion inflict pain on herself as punishment. Her friendship with Keiichi is marked by playful rivalry and mutual respect, and she values her bonds with other club members Rena Ryuuguu, Satoko Houjou, and Rika Furude, acting as a protective older figure for them. Her development across the story involves learning to reconcile her tomboyish exterior with her more vulnerable emotions, as well as grappling with the weight of her inherited duty and the mistakes of her past. In later arcs, she grows to accept help from others and becomes more open about her feelings, a shift that allows for a more hopeful resolution.

Notable abilities include her proficiency in martial arts, which she developed during her upbringing and which makes her a formidable fighter capable of overpowering trained opponents. She is also highly skilled at strategic games, using her sharp mind and psychological insight to gain an advantage. She is physically marked by the demon tattoo on her lower back, a constant reminder of her surrogate role as heir. Overall, Mion Sonozaki is characterized by a blend of strength, humor, duty, and hidden tenderness, making her a complex and stabilizing presence in the story.