Shioriko Mifune begins as a first-year student at Nijigasaki High School and later assumes the role of student council president after Setsuna Yuki. As the second daughter of the prestigious, wealthy Mifune family, she initially opposes the School Idol Club, challenging Setsuna's leadership through a re-election. She argues students should prioritize achievable successes over passions and explicitly states her presidency would disband the club.

This opposition stems from her sister Kaoruko's experience as a school idol whose group failed to reach Love Live. Shioriko once admired idols because of Kaoruko but concealed this passion, fearing similar failure. After engaging with club members, she confronts this fear, joins as the 11th member and 10th official school idol, becoming the first addition post-formation.

Her personality varies across media. The game depicts her as pragmatic, strict, and cold, delivering blunt assessments of others' capabilities. Despite initial friction, her effective advice eventually earns gratitude, revealing a genuine desire to help and connect. The anime portrays her as inherently kind, polite, and supportive from introduction, maintaining neutrality toward the idol club while assisting activities and compassionately protecting Setsuna's secret identity. Shared traits include calmness, academic excellence, and deep respect for Japanese traditions like traditional dance and flower arranging. She notably identifies individuals' strengths and weaknesses to guide them toward suitable clubs.

She belongs to the subunit R3BIRTH and volunteers at children's centers or neighborhood clean-ups. Physically, she has semi-long dark green hair, vermilion eyes, and often wears a small yellow hair ribbon, standing 160 cm (5'2") with a slim figure. Her birthday is October 5 (Libra, blood type AB). Her image color is jade green (#37b484), reflected in her penlight.

Notable details include wearing a kimono at home and sharing a childhood friendship with Lanzhu Zhong.