Yiyi Xu (徐依依, romanized as Shū Īī) is a Chinese scientist at Kyoto's Alltale Management Facility, collaborating with colleagues Tsunehisa Senko and Katagaki Naomi. Her technical contributions focus on stabilizing the facility's Homeostatic System, where her expertise remains vital despite not equaling Naomi Katagaki's exceptional skills.

Professionally rule-oriented and serious, Yiyi strictly follows protocols yet displays adaptability under peer pressure—such as joining an unauthorized barbecue during security lapses. When stressed, she instinctively reverts to speaking Chinese. Her personal habits include intruding into colleagues' rooms to perform meticulous 360-degree cleanings.

Physically, Yiyi has purple eyes and shoulder-length black hair, maintaining a skinny build. She consistently wears red lipstick, a red long-sleeve polo shirt, jet-black skinny jeans, red sandals, and a white researcher coat.

A defining moment arises when the Homeostatic System destabilizes. Yiyi joins four others in manually executing its shutdown to prevent uncontrolled rampage, directly intervening to halt its destructive activity and safeguard the facility's integrity.