Gekkō Kurenai is a sixteen-year-old student who serves as the president of the student council at Miyasaka High School. He is a young man with a calm and cold gaze emanating from his green eyes, and his jet-black hair frames a face that typically holds a curt, often offending expression. He openly declares himself a genius and maintains an outward superiority complex, an attitude that frequently pushes others away and leads him to label his council members using dismissive terms.

His past is defined by a traumatic event that occurred nine years before the story begins, an event involving his younger twin brother, Kurenai Hinata. Hinata was revealed to be the more naturally gifted of the two and, after experiencing apocalyptic visions that Gekkō could not see, sacrificed their parents to a demon. Before vanishing, Hinata vowed to return in nine years to consume Gekkō. This event instilled in Gekkō a fierce and relentless drive for power, a pursuit initially born from the fear for his survival that gradually transformed into a quest for vengeance.

Despite his arrogant and aloof demeanor, this mask hides a deep-seated loyalty to those he considers close. Beneath his tendency to treat his subordinates as servants lies a genuine regard for them as comrades and friends, individuals he would protect with his own life. His primary motivation is to gain enough strength to confront and eliminate his twin brother, viewing Hinata not as family but as a rival and an enemy responsible for their parents' murder and a series of assassination attempts.

In the story, Gekkō holds the position of the twelfth student council president, a role that also makes him the contracted guardian of the Holy Ground at Miyasaka High School. This position grants him the authority to manipulate dimensional gates that connect to other worlds. Furthermore, an ancient prophecy designates him as the Left Head of the Crow, a figure destined to kill the world-ending witch known as Ramiel Lilith, and he has been granted corresponding powers by entities known as the Tenma.

His relationships are complex and often adversarial on the surface. His contracted companion is Mirai Andou, an upper-class lightning demon descended from the Lightning Demon King Indra. Originally sent by Hinata to assassinate him, Gekkō overpowered Mirai and formed a pact that seals most of her abilities; releasing her true, more powerful form requires his explicit permission. He exhibits a possessive attitude toward her, declaring her his property despite her evident affection for him. His dynamic with the protagonist, Taito Kurogane, evolves from mutual dislike into a vitriolic partnership, where their constant bickering is underlaid by a growing sense of trust and brotherly care. He also develops unspoken romantic feelings for the ancient sorcerer Saitohimea, feelings he conceals behind a veneer of indifference.

Over the course of events, Gekkō shows development in his capacity for collaboration, learning to work as part of a team. At one point, he and Himea secretly collude to analyze and steal spells from a faction known as the Army, demonstrating a pragmatic alliance based on mutual respect despite their constant personal bickering. His interactions reveal that beneath the shared insults, he respects her skills and goals.

In combat, Gekkō wields an ancient sword of divine origin named Spell Error. The blade resembles an edgeless fencing rapier with a bell guard; it normally appears black but shifts to a blood-red color when activating its sealing powers. These powers can restrain targets or dispel magic, though they are ineffective against Tenma and Tenshi entities. To supplement his swordsmanship, he also uses traditional exorcism arts, spellcraft, and various talismans and charms for support in battle.