Shion Toumori is the shrewd student council president of St. Spica's Institute, one of three all-girls schools on Astraea Hill. She frequently schemes to advance Spica's interests within the Astraea Hill student governance structure. Despite this scheming nature, her personality remains moderate, distinctly avoiding extreme measures or obstructionism, unlike her associates Kaname Kenjō and Momomi Kiyashiki.

Her primary goal is ensuring a Spica student succeeds Shizuma Hanazono as the next Etoile, the student representative overseeing inter-school relations and ceremonial duties. She actively campaigns for prominent Spica student Amane Ōtori to pursue the position, persistently attempting to persuade the reluctant Amane. Shion capitalizes on school events to elevate Spica's profile; during cultural festival preparations, she proposes casting Amane as Don José opposite Shizuma’s Carmen to showcase Amane’s charm and bolster her Etoile candidacy. She also manipulates interpersonal dynamics, such as privately reassuring Hikari Konohana of her importance to Amane to strengthen Hikari’s resolve as a potential Etoile partner.

As Spica’s council president, Shion frequently clashes with Miyuki Rokujō, her St. Miator counterpart, during joint student council meetings. These disputes often involve Astraea Hill governance issues, including criticisms of Shizuma’s absenteeism from Etoile duties, revealing Shion’s assertive and animated side when advocating for Spica’s institutional interests.

Her name, Shion, combines the characters for "poem" (詩) and "sound" (音), while Tōmori derives from "winter" (冬) and "forest" (森).