Mion Sonozaki acts as class representative at Hinamizawa's school and leads the after-school games club. She projects a tomboyish, confident, and outgoing persona, frequently calling herself "oji-san" and trading crude jokes with Keiichi Maebara. Secretly, she harbors romantic feelings for Keiichi but struggles to express them directly due to shyness and fear of rejection, masking a hidden feminine side. Her competitive streak dominates club activities, where she often wins through strategic cheating or preparation, imposing embarrassing penalty games involving fetishistic costumes on losers. As the designated heir to the influential Sonozaki family, one of Hinamizawa's Three Families, Mion bears a demon tattoo signifying her status. This role demands a formal, authoritative demeanor during family duties, requiring her to suppress personal emotions to enforce traditions. She oversaw her sister Shion's punishment of fingernail removal for defying family rules, later inflicting the same punishment on herself out of guilt. Her upbringing included rigorous training in martial arts, firearms, and leadership, preparing her for responsibilities like attending village council meetings from a young age. Mion shares a complex history with her twin sister, Shion. They frequently swapped identities as children, culminating in a permanent switch on the day of the heir's tattoo ceremony: the younger twin (originally named Shion) assumed the identity of "Mion" and the heir role, while the older twin became "Shion," a swap unknown to the family. As a child, Mion actively protested the Hinamizawa Dam Project, engaging in violent clashes with construction workers. She founded the games club in 1982 to offer refuge to Satoko and Satoshi Hojo from their abusive aunt. Psychologically resilient, Mion is noted as the only club member who never succumbed to murderous insanity from Hinamizawa Syndrome in the original storylines, attributed to her logical mindset and sense of duty as heir. However, newer anime arcs *Gou* and *SOTSU* depict external manipulation causing her to exhibit violent behavior for the first time. Her relationship with Keiichi involves deep mutual respect evolving into implied romantic potential, highlighted in *Minagoroshi-hen* when he gifts her a doll acknowledging her femininity, and in *Miotsukushi-hen* where an accidental engagement leads to genuine consideration of marriage. In later timelines like *Rei* and the manga *Higurashi no Naku Koro ni Mei*, Mion appears as an adult who has married and has a daughter named Tamaki. She remains the family heir, succeeding her mother as head. *Taraimawashi-hen* depicts an alternate future after the Great Hinamizawa Disaster, showing Mion in her mid-20s suffering severe mental trauma and amnesia after a decade of hospitalization, resulting in lifeless eyes, diminished motor skills, and a hollow demeanor.