Kurou Daijuuji started his path as an archaeology student at Miskatonic University, a cover for his secret pursuit of sorcery under Dr. Henry Armitage. His life fractured when Wilbur Whateley assaulted him within the university's forbidden grimoire library, driving him to abandon his occult studies and drop out. He plunged into poverty as a private investigator in Arkham City, relying on Sister Leica Crusade for basic necessities like food. His fortunes shifted when Ruri Hadou of the Hadou Financial Group hired him to find a powerful grimoire, offering substantial payment. During this search, he encountered Al Azif, the physical manifestation of the Necronomicon, pursued by the Black Lodge. Forced into a contract with her through a kiss, he gained access to her vast magical knowledge and became the pilot of the imitation Deus Machina, Demonbane. This thrust him into the conflict between the Hadou Financial Group and Master Therion's Black Lodge, whose cruelty fueled Kurou's intense hatred.

He presents a cynical exterior yet possesses a strong moral compass. He frames his drive to help others as avoiding "a bad aftertaste"—the lingering regret of abandoning those in need—rather than idealism. This pragmatic sense of justice manifests as chronic hero syndrome, compelling him to intervene despite personal risk. He embodies a knight in sour armor, acknowledging the world's darkness while fiercely protecting remaining good. His constant financial instability and self-deprecating humor about his poverty contrast with his battle determination. Interactions with female characters, particularly Al Azif, often involve belligerent sexual tension, accidental pervert accusations, and jealous conflicts, though these relationships mature into deep bonds. Notably, mystical influences like Deep One gases can compromise his judgment, leading to near-assault incidents, though he regains control before causing harm.

His magical abilities flow primarily from his contract with Al Azif. This enables "Magius Style," a transformation granting superhuman physical attributes and flight via wing-like projections. His hair turns white, lengthens, and becomes weaponized; his eyes glow red. Key spells include Atlach-Nacha's web for immobilization, the Mirror of Nitocris for illusions, and the Scimitar of Barzai for energy blades. Later, he wields paired handguns channeling Great Old Ones: Cthugha (fire/plasma) and Ithaqua (ice/homing attacks targeting weaknesses). He demonstrates combat skills in swordsmanship and tactical analysis, often accelerating his perception against high-speed threats. Piloting Demonbane amplifies his power, enabling large-scale magic, space-time manipulation via Timaeus and Critias modules, and signature attacks like the Lemuria Impact detonating reality-erasing heat. The mecha also wields the Shining Trapezohedron for dimensional cutting and sealing. His bond with Al Azif grants resistance to mental, spiritual, and existential manipulation.

His development spans multiple timelines and endings. Initially reluctant toward the occult, witnessing Master Therion's actions fuels his resolve to destroy the Black Lodge. His relationship with Al Azif evolves from mutual utility to deep romance; he recognizes his love after her sacrifices, making their bond pivotal against nihilistic foes. In Al Azif's route, her temporary death leaves him a heartbroken badass operating with diminished power until her resurrection. Their partnership culminates in ascension as Elder Gods in some endings, wielding reality-altering abilities surpassing cosmic entities. Ruri Hadou's route sees him vanishing after a temporal conflict, returning amnesiac but rebuilding connections. Leica Crusade's route reveals her past as Metatron, with Kurou retaining memories of erased timelines to continue their relationship. Across all media, his journey moves from destitute detective to multiversal protector, retaining his "bad aftertaste" philosophy while gaining ripple-effect-proof memory that persists through cosmic retcons.