Rui Miyazumi is a college student affiliated with an unspecified institution, involved in demon investigation and exploration. He has short brown hair and an average physical build. On campus, he wears the standard school uniform: a short-sleeved white shirt, black pants, and a black tie. For missions or explorations in areas like Carceri, he dons a dark form-fitting combat suit optimized for tactical engagements. His relationship with Yuno Amase is notably close, with multiple instances strongly suggesting romantic involvement. This includes observations of them flirting at the Post Exchange and Rui openly complimenting Yuno when she modeled a swimsuit during a visit to the OS Base hot spring. During joint operations, he consistently addresses Shizuri Castiella Kasugaya as "Squad Leader," indicating a formal chain of command. Rui specializes in combat as a skilled sniper, utilizing custom Spell Fragmentation rounds. These rounds deploy ritual spells capable of immobilizing powerful entities such as Shinigami and Golems. Against demons in broader engagements, he employs heavy pistols and a carbine loaded with varied ammunition, including rubber bullets for less lethal force and enhanced rounds for greater effectiveness against supernatural targets. In the storyline "The Broken Holy Spear," Rui investigates reports of missing workers alongside Yuno Amase within New Itogami Island's Sixth Cluster sector, demonstrating his role in field investigations and response efforts.