Sayo Shinoyama is the second eldest granddaughter of Rozan Shinoyama, wealthy patriarch of the Shinoyama financial conglomerate. She resides in the family mansion, maintaining an unemployed NEET status sustained by family wealth. Her name combines the characters for "four" (四, shi), "from" (乃, no), "mountain" (山, yama), "small" (小, sa), and "night" (夜, yo). She possesses long, light-colored hair, her bangs perpetually covering her eyes. At family meals, she pushes a sleep mask back over her hair. Sayo exhibits an obsessive focus on shark-themed media, particularly the "Sharkborg" film franchise and titles like *The Shallows*, *47 Meters Down*, and *Bait 3D*, qualifying her as an otaku specializing in cinematic sharks. She deflects questions about her unemployment by claiming a role in "targeted security for a single room". Her narrative prominence increases during the Shinoyama Mansion Fire Arc. During a meal attended by Polka Shinoyama (inhabited by the Corpse God) and Misaki Sakimiya, Sayo recognizes the design of a stuffed shark on Polka's jacket as originating from *Sharkborg Part 2*. She delivers an impassioned critique of the franchise, oblivious to the Corpse God's unfamiliarity. Later, she recommends shark films to Rozan, who acknowledges enjoying her prior suggestion of *The Shallows*. A fire then engulfs the mansion, destroying Sayo's bedroom and roughly half her shark memorabilia collection. This loss plunges her into prolonged depression. Due to her displacement and distress, Rozan assigns security specialist Lei Xiaoyu as her escort and relocates them both to Polka's residence. Sayo arrives carrying a shark-themed sleeping bag and an oversized backpack, remaining withdrawn while Lei handles introductions via a video message from Rozan. She spends her first night sleeping despondently on the floor in her sleeping bag. The next day, her melancholy persists despite others' attempts to accommodate her, though plans are made to prepare a dedicated room for her and Lei. Her character exhibits no significant development beyond this relocation. Her interests remain confined to shark films, with no documented engagement in broader activities or relationships outside her fandom.