Shannon serves the Ushiromiya family on Rokkenjima as a maid, using this name as a pseudonym while her true identity is Sayo Yasuda. Created by Kinzo Ushiromiya as "furniture" – a term for servile beings – she was uniquely granted a heart. Kinzo believed this endowment imbued her with protective magical abilities, specifically barrier magic.

Born from an incestuous union between Kinzo and his daughter Beatrice, she was initially named Lion Ushiromiya. As an infant, Natsuhi Ushiromiya threw her off a cliff, inflicting severe injuries that rendered her infertile and incapable of sexual intercourse. Rescued by the servant Genji and physician Nanjo, she was raised under the alias Sayo Yasuda at the Fukuin House orphanage to hide her origins. She started working on Rokkenjima at age nine, facing ostracism from older servants. To cope with this isolation and her fractured identity, she manifested multiple personas, including Shannon and the male servant Kanon, whom she regards as a brother.

Physically, Shannon presents as a teenage girl with short brown hair, blue eyes, and notably large breasts, later revealed to be artificial padding. She wears a maid uniform with a white beret and bears the Ushiromiya crest tattooed on her left thigh. Her personality is gentle yet prone to clumsiness, especially when nervous. She struggles deeply with self-worth due to her "furniture" status, believing herself unworthy of human relationships.

Her story involves romantic relationships with George Ushiromiya, whom she eventually accepts as her fiancé, and Battler Ushiromiya, her childhood love. Battler's six-year absence led her to suppress her feelings for him, though he retained a protective soft spot for her. George's marriage proposal intensified her internal conflict, particularly regarding her inability to bear children or engage physically, fearing it would disillusion him. Beatrice exploited this insecurity, offering Shannon a golden brooch to guarantee George's love in exchange for destroying a spirit mirror that suppressed Beatrice's power.

Magically, Shannon specializes in barrier creation, capable of repelling attacks, atomizing matter, and disrupting spiritual energies. Her powers amplify significantly when synchronized with Kanon, as they originate from the same being. Together, they deploy magic interference fields and counter high-velocity projectiles.

Across the episodic narratives on Rokkenjima, Shannon experiences multiple deaths and resurrections. Key moments include her murder in the storehouse during the first twilight in "Legend of the Golden Witch" while wearing George’s engagement ring. In "Turn of the Golden Witch," she attempts to fight Beatrice alongside George and Gohda but is killed. "Banquet of the Golden Witch" depicts her as a first-twilight victim, while "Alliance of the Golden Witch" shows her captured and later killed by the Chiesters after escaping. During meta-narrative segments, particularly in "Requiem of the Golden Witch," she interacts with the detective Willard H. Wright, hinting at her shared existence with Kanon.

Her relationship with George culminates tragically in "Turn of the Golden Witch," where she likely confronts him about her insecurities or proposes a lovers' suicide. George’s rejection or unexpected response leads her to kill him before dying herself. In later fragments, her bond with Battler resurfaces when he defends her after learning she is Beatrice’s human identity, offering to share her burdens.

Trivia includes the unused surname "Ronoue" (implying a relation to Genji), the numerical wordplay within her true name "Sayo" (combining the kanji for "three" and "four," referencing the 34th demon Furfur), and her role in the fighting game "Ougon Musou Kyoku," where her moveset involves magical barriers and summoning seagulls.