"The Blood" designates Kojou Akatsuki, the Fourth Primogenitor, whose evolution defines the series. Once an ordinary high school student, his transformation ignited during a terrorist assault on Fairy's Coffin, where he shielded his sister Nagisa and Avrora Florestina. Mortally wounded, Avrora transformed him into her blood servant and commanded him to destroy her, transferring her powers and memories. Though initially lost, fragments of these memories resurfaced during battles with entities like Giada, the Third Primogenitor disguised as Avrora. Yukina Himeragi later performed a ritual restoring his memories completely. Kojou presents as a 16-year-old with dark silver-blue hair and blue eyes that shift crimson under emotional duress or anger. His vampiric nature necessitates avoiding daylight, resulting in fair skin. He typically wears the Saikai Academy uniform, often layered with a hoodie. Enhanced vampiric traits grant superhuman strength, speed, reflexes, potent regeneration, and immortality, enabling recovery from injuries like decapitation or incineration. Additional abilities include mist transformation, specific-condition precognition, and "Cleansing," a reality-warping spell derived from Cain the Sinful God. His Primogenitor status manifests through twelve Beast Vassals, powerful familiars central to his power. Initial struggles controlling them stemmed from his refusal to drink human blood, causing rebellion. Mastery required consuming blood from his blood servants' rings and directly from Avrora and Yukina. Each Beast Vassal corresponds to a zodiac sign with unique abilities: * **First Familiar**: Mesarthim-Adamas manipulates diamonds. * **Second Familiar**: Cor Tauri-Sucinum controls lava. * **Third Familiar**: Al-Meissa Mercury consumes spatial dimensions. * **Fourth Familiar**: Natra-Cinereus renders objects ethereal via mist. Later developments included summoning Shikigami through talismans, though early attempts caused instability. Kojou's personality blends inherent kindness and empathy with a quick temper, especially when loved ones are threatened. His leadership charisma draws allies and converts adversaries into comrades. He exhibits "siscon" tendencies, prioritizing his sister Nagisa, and often remains oblivious to romantic feelings from others like Yukina and Asagi Aiba. This dynamic evolves, with glimpses indicating future marriages to both Yukina and Asagi. Within *Strike the Blood IV*, pivotal arcs include "Reunion of Vampire Princess" and "Seize the Twelve Beast Vassals." When his father retrieves Nagisa for purification rites, Kojou encounters Tobias Jagan and learns of Dimitrie Vatler's role in a Chaos Zone civil war. Vatler sends Celesta Ciate, a girl linked to the conflict, prompting Kojou to investigate her origins with allies like Astarte and Kanon Kanase. Later, after Nagisa's corrupted magic circle image surfaces, Kojou confronts threats tied to his Primogenitor legacy, achieving full command over his Beast Vassals without blood consumption due to Avrora selecting him as her successor.