Shion Sonozaki, the elder twin daughter of Hinamizawa's powerful Sonozaki family, permanently swapped identities with her younger sister during the family heir's demon tattoo ceremony. Consequently, her sister assumed the name Mion and the heir's role, while she became Shion and lived as the younger sibling. Bound by traditions favoring a single heir, Shion was sent to St. Lucia's boarding school. She escaped in 1982 with help from Sonozaki bodyguard Tatsuyoshi Kasai. The family permitted her residence in the neighboring town of Okinomiya on the condition of discretion. Shion typically displays a more refined, feminine demeanor than Mion, accentuated by her St. Lucia education, yet harbors a rebellious, manipulative, and impulsive nature. She openly despised St. Lucia, calling it a "factory for manufacturing ladies," and meticulously studied its security to engineer her escape, manipulatively spreading rumors about an affair with a male teacher. She works as an Angel Mort waitress and assistant manager for the Hinamizawa Fighters baseball team. Her appearance mirrors Mion's mint-green hair and turquoise eyes, but she distinguishes herself with a half-ponytail tied by a yellow ribbon. Her personality blends protectiveness with volatility. She developed deep affection for Satoshi Houjou after he defended her from delinquents, supporting him by securing a stuffed bear for his sister Satoko. After Satoshi vanished during the 1982 Watanagashi Festival, Shion's grief and jealousy, amplified by Hinamizawa Syndrome—a parasitic infection causing psychosis—manifested destructively. In Meakashi-hen, she orchestrated the murders of villagers, including Rika Furude and Satoko, believing them responsible for Satoshi's suffering, culminating in suicide after confronting Mion. Conversely, in arcs like Minagoroshi-hen and Matsuribayashi-hen, Shion honored Satoshi's request to protect Satoko, forming a sisterly bond. She risked her life saving Satoko from her abusive uncle, Hōjō Teppei, initially threatening lethal violence before cooperating with Keiichi Maebara for legal intervention. She also aided friends against paramilitary forces using non-lethal tactics like a stun gun, demonstrating growth in emotional control. Shion's relationship with Mion remains complex; they share a close bond and occasionally impersonate each other, yet their roles create tension. Shion envies Mion's heir status and resents the family's oppressive traditions. Despite this, she consistently calls Mion "Sis" (Onee) and expresses a desire to be reborn as twins with her in another life. Her interactions with Keiichi involve teasing and conflicting emotions, viewing him as both a perverted substitute for Satoshi and a means to provoke Mion's jealousy. In the Rei manga, set 35 years later, Shion marries Satoshi. Her susceptibility to Hinamizawa Syndrome stems from prolonged village absence reducing her parasite immunity, compounded by traumatic experiences like familial torture—including fingernail removal for defiance—and unresolved grief over Satoshi. Rika Furude observed that Shion overcoming her "inner demon" depended on rejecting jealousy and embracing her protective role toward Satoko, pivotal in averting tragic outcomes in later arcs.