Kotarou Tennouji presents as a lighthearted, sociable high school student, yet this demeanor conceals profound complexities. Prior to the narrative, a significant accident triggered memory issues and transformed his personality from cold isolation to outgoing sociability, driving him to forge connections through ventures like founding the Occult Club. He possesses latent supernatural abilities: the primary power "Rewrite" enhances his physical capabilities—boosting strength or nullifying toxins—by consuming his life force and humanity. Each activation hastens his metamorphosis into an immortal familiar, marked by a visible progression indicator. His secondary ability "Aurora" crystallizes blood into energy constructs like claws, bullets, or shields, with excessive use depleting his blood reserves as a tactical constraint. Kotarou's development branches dramatically based on choices, manifesting distinct personas—a justice-driven idealist in one path, a devoted protector in another, or a pragmatic strategist elsewhere. His core adaptability reshapes goals and ethics through alliances, whether guarding Akane or confronting moral compromises in the Terra route. Critical junctures include a near-fatal encounter binding him as Kotori's pseudo-familiar and a climactic sacrifice of personal ties for survival. In the comedic crossover Kaginado, he retains core traits within a chibi aesthetic, interacting with Key franchise characters during absurd escapades like leading heroines into space within a luminous orb. Rewrite briefly fuels physical comedy, such as enduring a vibrating soccer ball in club competitions. His design features spiky light-brown hair, orange eyes, and a standard school uniform adorned with a crown-shaped brooch. Casual wear favors layered jackets and practical accessories like a bicycle or camera. Story conclusions diverge across media: one ending strips his powers permanently, while another evolves him into a full familiar contracted to multiple heroines. The Terra route culminates as Earth's guardian, fulfilling a promise rooted in his earliest supernatural encounters.