Shizuri Castiella Kasugaya possesses a fair complexion, long white hair, blue eyes, and two white horns usually hidden beneath a wimple. These horns function as sensory organs detecting auras and demonic energy, though physical contact induces sexual stimulation. She frequently wears the College of Magical Arts uniform: a white shirt, black tie, and purple skirt. Her personality demonstrates bravery and strong leadership. She exhibits profound inexperience in romantic matters, comparable to an elementary schooler, and harbors a fondness for tea and coffee, especially when poured by Kojou Akatsuki. Shizuri first appears on Onrai Island's shore, encountering the amnesiac Kojou Akatsuki. Identifying him as the Fourth Primogenitor—an immortal vampire with twelve destructive beast vassals—she declares herself his watcher, pointing her vermilion sword at his neck. During a training mission against an armored Shikigami, she assigns Kojou passive support due to his inability to access his powers. When Kojou is severely injured protecting her team, she reacts with intense anger and later remains at his hospital bedside, blaming herself. Her relationship with Kojou evolves romantically. She becomes flustered when teased about women he encountered and displays jealousy during his interactions with Yukina Himeragi. She accepts an engagement ring from Kojou and eventually becomes his wife in confirmed future timelines. In the "Broken Holy Spear" arc, she joins Kojou and Yukina to form the Demon Sanctuary Research Club, experiencing tensions over meeting locations. She later assists Kiriha, a priestess of the Bureau of Astrology, in slaying a demon beast. Instinctively perceiving Kiriha as dangerous—comparing it to a "wild wolf encountering an unfamiliar human"—Shizuri is recognized by Kiriha as an ogre in return. Her abilities include duel-wielding, Shikigami manipulation, and enhanced strength, durability, speed, and agility. She wields a silver mace and the vermilion blade Hauras, accompanied by the chant: "This fang is the light rending our darkness...". The sword previously belonged to Shinako Kasugaya, from whom Shizuri derived her surname. Shizuri taught Kojou Akatsuki to read and write English, leading to cheating suspicions when he performed well on an English test. Age restrictions in attack mage labor laws meant she lacked an official attack mage license during her early appearances. She also requires assistance operating newer vending machines.