Mion Sonozaki is a central character in Higurashi: When They Cry. She is a fourteen-year-old student at the Hinamizawa schoolhouse, where she serves as class president and the leader of the after-school games club. Mion is the twin sister of Shion Sonozaki and is recognized as the heir to the Sonozaki family, one of the three influential families that hold considerable power over the village of Hinamizawa. A demon tattoo on her lower back, traditionally given to the firstborn heir, marks her as the future head of the family, though this branding is the result of a childhood name swap between the twins.

Mion's background is deeply tied to the Sonozaki family's yakuza roots and their control over the village. Raised by her grandmother, Oryou Sonozaki, she underwent extensive training in martial arts, cooking, sewing, flower arrangement, and even helicopter piloting as part of her preparation for the family headship. She was an active participant in the Dam War, the conflict that erupted when the government attempted to build a dam over Hinamizawa, and was arrested multiple times for her role in the protests.

In terms of personality, Mion is cheerful, energetic, and highly sociable. She has a boisterous tomboyish demeanor, frequently referring to herself as an old man or a friendly uncle. She thrives on competition and enjoys organizing games for her club members, often employing clever tactics and bending the rules to secure victory. Despite her wild and confident exterior, she has a more reserved and girlish side that she struggles to show openly, particularly around her crush, Keiichi Maebara. When acting in her capacity as the Sonozaki heir, she becomes serious, formal, and almost expressionless, setting aside her playful nature to fulfill her responsibilities.

Mion's motivations are rooted in her loyalty to her friends and her duty to her family. She values her clubmates deeply and is willing to put herself in danger to protect them. Her role in the story is that of a grounding presence and a natural leader. She is one of the few main characters who consistently resists the paranoia and violent impulses brought on by the Hinamizawa Syndrome, never committing murder or becoming dangerously violent across any of the story's arcs. This restraint makes her a stabilizing force amid the chaos that unfolds.

Key relationships define much of her arc. She harbors romantic feelings for Keiichi Maebara, which she expresses indirectly through teasing and rivalry. Her dynamic with her twin sister Shion is complex and consequential. The twins swapped names as children, which led to Mion being branded as the heir while Shion was treated as the spare. Mion carries guilt over this outcome and often defers to Shion out of that sense of responsibility. Within the club, she acts as a leader and older-sibling figure to Rena Ryuuguu, Satoko Houjou, and Rika Furude, fostering a close-knit group that forms the emotional core of the story.

Mion's development across the narrative involves a gradual exploration of her dual identity. She balances her carefree school life with the weight of her family obligations, and she learns to reconcile her private vulnerability with her public strength. Her competence in martial arts and strategic games is a defining ability, and she is shown to be an effective commander of people, at one point outmaneuvering trained soldiers through tactical insight. She also collects board games and card games and devises custom rules for the club's activities. A notable skill is her proficiency with an airsoft gun, which she carries in a shoulder holster as part of her personal style, though she does not rely on it in serious confrontations. Her emotional resilience and refusal to succumb to the paranoia that afflicts others stand as her most significant quality, marking her as a unique presence among the main cast.