|Occupation||Waitress at Angel Mort|
Phantom manager of Hinamizawa Fighters
|Likes||Satoshi Houjou, cream croquettes, Mont Blancs|
|Dislikes||Canned food, rules|
The name "Mion" along with the headship of the Sonozaki family was reserved for the first-born of the Sonozaki sisters. Because of this, Shion was born as "Mion Sonozaki" as she was born first. Furthermore, her sister Mion was originally named "Shion Sonozaki". As the firstborn, Shion was made to attend family meetings to prepare for the headship. Mion found this unfair and proposed that the twins occasionally swap names so that both could attend family meetings. However, the twins swapped names on the day that the first-born was to be branded with an oni (鬼, demon) tattoo. As a result, Mion was permanently, and falsely, branded as the firstborn heiress to the Sonozaki family. She permanently adopted the name "Mion" while her sister adopted the name "Shion". No one other than the twins appears to be aware of this fact.
As a child, Shion attended St. Lucia Academy, a boarding school for girls. Shion always hated St. Lucia and called it a "factory for manufacturing ladies". She run away from the school in both 1982 and 1983 with the help of her bodyguard, Tatsuyoshi Kasai. While in Hinamizawa, Shion fell in love with Satoshi Houjou. Satoshi was frequently abused by his aunt in place of his sister, Satoko Houjou. Because Satoshi always protected Satoko, Shion began to blame Satoko for Satoshi's suffering at home. As a result, Shion came to despise Satoko. However, after Meakashi-hen, Shion develops a sister-like bond with Satoko due to a promise made with Satoshi to protect Satoko in his absence.Shion works at the Angel Mort restaurant and serves as the manager's assistant for the Hinamizawa Fighters.
Shion appears as a pretty young girl with green hair adorned with a yellow ribbon to differentiate her from her sister. Shion's outfits vary depending on the artist. In her original incarnation, at school, Shion wears a grey blazer over a white shirt and red tie. Her casual clothing consists of a white sleeveless turtleneck sweater with a golden necklace and a dark green buttoned skirt. In Alchemist sprites, at school, Shion wears a white shirt, a grey tie and a short, pleated blue skirt. Her casual clothing consists of a white spaghetti strap blouse over a black blouse and a short blue skirt.When working at Angel Mort, Shion wears the standard Angel Mort uniform, a somewhat complex red, white, and blue uniform with a collar and thigh-high boots.
Shion is much "girlier" than her sister. As a result of her upbringing at St. Lucia Academy, Shion appears to be quite refined and proper. However, Shion is incredibly shrewd, cold, manipulative, and delinquent, more so than the tomboyish Mion. This is evident in her escape attempts from St. Lucia. Shion studied all of the school's security systems, guard schedules, and staff behaviors and managed to manufacture a rumor that she was involved with a male teacher. In Meakashi-hen, Shion manages to manipulate the entirety of Hinamizawa by stealing Mion's identity and taking advantage of the town's escalating paranoia.
Despite her cold personality, Shion has a very supportive personality and appears to be just as friendly as her sister. Her love for Satoshi is very transparent and she frequently helps him when he's in trouble. Shion's strong feelings lead her to become envious very easily, contributing to her heightening Hinamizawa Syndrome. This is the inciting reason for her breakdown in Meakashi-hen. Hearing Mion talk about Keiichi Maebara awoke her "inner demon" as she became extremely jealous that Mion could have a living crush nearby while Shion's own crush had disappeared. Shion also becomes jealous of Mion's status as heiress to the Sonozaki family, Rika Furude's status as the reincarnation of Oyashiro, Satoshi's undying love for Satoko, and Rena Ryuuguu's "inner demon" which Shion felt was far more fearsome than Shion's own "inner demon".
- The "on' in Shion's name is written with the character "音" meaning "sound". The same character is in Mion's name as well as every servant who worked at the Fukuin House such as Shannon and Kanon.
- Other than Satoko Houjou, Shion is the only character in Higurashi no Naku Koro ni with Hinamizawa Syndrome who never scratches her throat.
- In Higurashi Daybreak, Shion often yells "Ora, ora, ora" when attacking. This is a reference to Jotaro Kujo from Jojo's Bizarre Adventure.
- Shion has a fear of canned foods, a result of Tatsuyoshi Kasai telling her stories when she was young about canned food containing human meat.