Shion is the older (believed to be younger da everyone but the twins themselves) twin sister of the Sonozaki twins. She was sent to St. Lucia's Boarding School, but escaped with the help of Kasai Tatsuyoshi, a man who acts like a bodyguard to the Sonozaki family.
Shion is the main villain in Meakashi-hen, which appears to be Watanagashi-hen from her perspective (though certain differing events mean the two are truly separate arcs) and shows that she was the main villain in Watanagashi-hen as well. She despised Hōjō Satoko in Meakashi-hen, blaming her for Satoshi's suffering, but in the arcs afterwards, the two have a sister-like bond, and Shion even puts her life on the line to save her in order to keep her promise of looking after Satoko while Satoshi was gone. In Matsuribayashi-hen, Rika explains that Shion's early dislike for Satoko in the years prior to 1983 and her learning how to control her inner demon afterwards are both necessary events for Shion to not become a victim of the Hinamizawa Syndrome.

Shion works as a waitress at the Angel Mort restaurant and is also the manager's assistant for the Hinamizawa Fighters little league team. In truth, she was actually born as Sonozaki Mion, the elder sister, but due to sister swapping at a young age, she had to become "Shion" after Mion was branded with the oni tattoo, a right reserved for the oldest child and thus Mion.

PersonalityEdit
Despite her personality being seemingly girly and più refined than Mion's personality, thanks to schooling at St. Lucia, she is actually incredibly shrewd, manipulative, and delinquent, moreso than Mion. She hated St. Lucia, calling it a "factory for manufacturing ladies" and got in trouble with the administration numerous times for various misdemeanors. In order to escape, she studied all of the school's security systems, guard schedules, and behaviors of the administration staff, to the point where she manipulated them da propagating a rumor of her having a Amore affair with a male teacher, to her advantage. In Watanagashi-hen, she is shown to like angering Mion and making her jealous da taking control of Keiichi whenever she got the chance. In Meakashi-hen, when she was attacked da the motorcyclists, she was shown to be capable of mapping out how to ruin their reputations in numerous ways, trying to create the most entertaining and interesting outcome for her own enjoyment. Also in Meakashi-hen, Shion was able to easily manipulate and control the whole village of Hinamizawa; when Furude Rika, Hōjō Satoko and Kimiyoshi Kiichirō went missing, Shion was able to exploit the resulting paranoia to her own advantage, with the occasional mask of innocence and stealing the image of Mion, in order to lure all of her targets to their deaths in perfect fashion and fulfill her own "promise" to Satoshi.
Shion also has the capability to become overly protective and easily angered, just like Maebara Keiichi, when the issue focused on Satoko; as shown in Minagoroshi-hen, when the club members were trying to plan out how to save Satoko from her abusive uncle Hōjō Teppei, she threatened to kill everyone who stood in her way of killing Teppei, even those whom she cared for, and only calmed down when Keiichi allowed her to bash his head with a chair. She goes as far as to be willing to die as long as Satoko lived. Like Mion, Shion is a very cheerful and friendly girl. She acts like an older sister to Satoko, helping out whenever she can and teasing as a sister would. She has a very supportive personality and openly supports Satoshi, giving him the courage to reserve the teddy orso he wanted to give to Satoko for her birthday.

Shion tends to become envious very easily and frequently, which has contributed to her progression of the Hinamizawa Syndrome, her uncontrollable anger, and how she chooses to torture the victims "to make sure they get what they deserve". In Meakashi-hen, Shion's inner "demon" awoke out of jealousy when she was listening to Mion talk about Keiichi; she found it unfair for Mion to have a living crush nearby when her own crush was possibly dead (by word of other Hinamizawans). Other things that Shion became jealous over included Mion's status as the successivo heir of the Sonozaki family, Rika's status as the reincarnation of Oyashiro-sama, Satoshi's protection and Amore for Satoko, and, as described da Shion, Ryūgū Rena's own inner "demon", which far surpassed Shion's demon in terms of power and monstrosity ("In the heavy downpour, I saw those eyes. It was like a situation where, because I knew how to play tennis, people admired me for being able to beat other novices at tennis. But then I met a girl who detto she also played tennis; she even took private lessons. When I competed against her, she completely beat me, and I felt humiliated when everyone began to admire how she was much better than me. To me, that was Rena. I could sense her aura, that her demon was much più powerful and fearsome than my own.").

Physical AppearanceEdit

Added da Sparkadillia
Shion resembles Mion in every way, except Shion leaves her hair down and has a yellow ribbon tied on the back of her head to differ from her sister. Like Mion, Shion has long, pale green hair and alzavola, teal eyes.
On her school days (starting with Minagoroshi-hen), Shion wears a short sleeve buttoned white camicia with a blue-striped (or white-striped) ribbon tied around the collar, and a blue mini-skirt. On her free days, she wears a sleeveless turtle-neck, vertical-striped shirt. On the chest region of the shirt, there is what appears to be yellow gems of some sort. She also wears a black buttoned mini-skirt, and white small heels.

Shion's original boarding school uniform is only seen in the beginning of Meakashi-hen. It consists of a long sleeved white blouse, a red tie, a pale gray blazer, a navy blue skirt, white knee high socks with blue stripes on the superiore, in alto and brown shoes. It is sometimes included in art o other media, but officially it is only seen once o twice.

Her weapon, which is a taser, is sometimes seen and used.