Yurio Mizukami is a third-year student at Jigen High School, boasting fair skin and a generally gloomy, sickly demeanor often associated with anime enthusiasts. He leverages his youthful, pre-pubescent appearance as an amateur magazine model, maintaining a social media following exceeding 20,000 Instagram users.

His past holds a life-altering motorcycle accident involving fellow student Tsuyoshi Kikawada. Yurio secretly rigged Tsuyoshi's motorcycle hoping to create a chance for friendship, but unintentionally caused the crash that left Tsuyoshi seriously injured and academically held back. Yurio subsequently helped Tsuyoshi recover, leading Tsuyoshi to view him as his "angel," while Yurio kept his role in the accident a secret, creating underlying tension.

During remedial lessons, Yurio and his classmates, along with their teacher, were transported to an alternate dimension by Spudio the 22nd and forced into puzzle battles against Sphinxes. Answering one riddle incorrectly resulted in Yurio being consumed by a Sphinx. Though his classmates later rescued him by solving a puzzle, the consequence of his consumption was the erasure of the person most precious to him: Tsuyoshi Kikawada.

Determined to reverse this, Yurio dedicated himself to intense study and preparation for future Sphinx encounters. His efforts proved crucial when the group successfully solved two consecutive puzzles against the Sphinx Paramesos, defeating it and restoring Tsuyoshi's existence. Following this victory, Yurio confessed his culpability for the motorcycle accident. Tsuyoshi forgave him, and this honesty solidified their bond, forging a closer friendship.