Affiliated with the Anglican Church's 0th Parish, Necessarius, magician Stiyl Magnus specializes in rune and fire-based magic. Despite a mature appearance, he is 14 years old, towering at approximately 200 cm. His shoulder-length blonde hair is dyed red, complementing his red eyes and a distinctive barcode tattoo beneath his right eye. He consistently wears a black priest robe, silver rings on all ten fingers, earrings, and a cigarette – reflecting his heavy smoking habit and stated view that a nicotine-free world would be hell. The high energy demands of his potent spells contribute to his noted physical weakness.

Raised by the Anglican Church, likely as an orphan, he trained in magic and became Index's partner alongside Kaori Kanzaki. He traveled with Index to copy grimoires from her mind and accompanied her to the Vatican for further book implantations. During this time, he developed strong protective feelings for her, promising to safeguard her at all costs. However, he and Kanzaki were informed by the church that Index required annual memory erasures to survive, her brain overwhelmed by 103,000 grimoires and her photographic memory preventing natural forgetting. Despite seeking alternatives, they complied. They wept each erasure as Index lost memories of their friendship. After each reset, they attempted to rebuild bonds, but the emotional burden led them to cease efforts. This allowed Index to mistakenly view them as enemies seeking her grimoires, a detachment strategy intended to spare her recurrent pain. Stiyl acquired his barcode tattoo, piercings, and rings after the first memory erasure.

Stiyl exhibits a gruff, austere demeanor with a short temper and no-nonsense attitude, often reacting violently to provocations. He harbors deep care for Index, frequently becoming embarrassed when this is noted. His greatest regret is failing to save her without memory loss, leading him to disobey church capture orders upon discovering the erasures were unnecessary and based on a lie. He resents Academy City for its restrictions on minors buying cigarettes and displays pettiness and jealousy toward Kamijou Touma due to Touma's closeness with Index, though he respects Touma's abilities and acknowledges Index's happiness with him. Stiyl often causes Touma discomfort during collaborations but tolerates him for Index's sake. He demonstrates loyalty to comrades and a capacity for kindness, as seen protecting Patricia Birdway from Richard Brave's schemes.

Stiyl's magical expertise centers on rune magic. He deploys tens of thousands of laminated, waterproof rune cards via magic, not manual placement. His primary focus is fire manipulation, with flames burning at 3000 degrees Celsius. Key spells include the Flame Sword, formed from red or bluish-white flames and used in techniques like Squeamish Bloody Rood (scissor-like dual-sword strikes), and Innocentius, a regenerating 3000-degree fire monster summoned through lengthy incantation and rune cards. Innocentius requires significant energy, weakening Stiyl physically, and operates within a radius defined by the cards' placement—potentially spanning kilometers with extensive preparation. He employs runes for diverse effects: Kenaz for ignition, Ansuz for communication, Opila for creating "empty space" fields or asset transference, and others for restraints, illusions, acid generation, or extinguishing flames. He manipulates heat to create mirages, heals burns, and uses cigarette smoke for symbolic messaging or mental analysis, as demonstrated diagnosing Patricia Birdway's condition.

His chronological involvement begins with the Index Arc, where he and Kanzaki attempt to retrieve Index for memory erasure. After initial conflict with Touma, he observes Index's happiness and later assists Touma upon learning the church's deception. In the Deep Blood Arc, he returns to Academy City to handle alchemist Aureolus Izzard, coercing Touma's help and protecting Index with Innocentius. He later alters Aureolus's appearance after the alchemist loses memories and powers. During the Orsola Aquinas Rescue Arc, collaborating with Agnese Forces under Laura Stuart's orders, he "kidnaps" Index to secure Touma's aid and later fights alongside him and the Amakusa Christians against Roman Catholic nuns. In the Daihasei Festival Arc, he partners with Tsuchimikado Motoharu to intercept Oriana Thomson's exchange of the Stab Sword, utilizing Touma as bait and enduring counterspells. Throughout, his commitment to Index's well-being and complex rivalry-alliance with Touma remain consistent.