Kojou Akatsuki, a 16-year-old high school student, is thrust into a supernatural destiny when he transforms into the Fourth Primogenitor—a legendary vampire of immense power—after shielding his sister Nagisa and Avrora Florestina during a terrorist attack. Avrora’s bite erases his memories of the incident and their bond, though fractured recollections emerge during battles, such as confronting Giada, the Third Primogenitor masquerading as Avrora, and through Yukina Himeragi’s ritual, which fully restores his past. Before his vampiric awakening, Kojou was a skilled basketball player, leading his middle school team to a city championship and earning MVP honors. A career-ending injury and his teammates’ dependence, which contributed to a crushing defeat, pushed him to abandon competitive sports, though he occasionally plays recreationally. His fierce protectiveness toward Nagisa, rooted in her chronic health struggles, shapes his choices—from risking his life to retrieve her cherished childhood hat to quelling his own violent outbursts when reminded of her safety. As the Fourth Primogenitor, Kojou commands twelve familiars, embodiments of his vampiric might. Initially rebellious due to his refusal to consume human blood, they submit after he drinks from allies, a need later nullified by Avrora’s influence. Notable familiars include Mesarthim-Adamas (diamond creation), Al-Meissa Mercury (a serpent devouring space), and Sadalmelik-Ablus (reversing phenomena), each tied to constellations and granting unique combat advantages. His vampiric traits confer immortality, rapid regeneration, heightened physical prowess, and sporadic precognitive flashes under duress. Kojou balances kindness, empathy, and a laid-back attitude with a volatile temper when challenged. Driven by an unwavering moral code, he defends others relentlessly, often converting foes into allies through sheer charisma. Though initially oblivious to romantic advances from Yukina, Asagi Aiba, and others—due to his singular focus on Nagisa—he gradually acknowledges their feelings as bonds deepen. These relationships evolve into blood-servant pacts, akin to marital vows, with Yukina and Asagi each bearing future children: Reina and Moegi, respectively. Critical connections include Yukina, his former watcher turned trusted partner and wife; Asagi, a tech prodigy and childhood friend harboring unspoken affection; and Avrora, whose soul fragment within Nagisa sparks Kojou’s fierce loyalty. His journey weaves through reclaiming lost memories, mastering familiars, and clashing with existential threats like Cain, progenitor of demons. Reluctant to embrace his vampiric role initially, Kojou grows into a decisive leader, eventually spearheading "The Empire of The Dawn." Through trials and alliances, he shifts from a reactive youth to a strategic guardian, safeguarding both his loved ones and the supernatural world.