Kotarō Izumi, the sixteenth-generation head of the prestigious Izumi household, has brown hair and olive green eyes. As a child, he often wore a white yukata, a style he maintains into adulthood. In his youth, he discovered an abandoned Hybrid Child at a garbage dump and brought it home, naming it Hazuki. Despite repeated attempts by his family to dispose of the Hybrid Child and replace it with a new one, Kotarō stubbornly insisted each time that it had to be Hazuki, refusing to accept a substitute. Hazuki serves as Kotarō’s tutor, butler, and caretaker. Kotarō is an unreliable family heir who often fails exams and fabricates excuses to avoid studying. He expresses frustration that Hazuki grew taller than him, considering it unjust. His character is marked by stubbornness and childish tantrums, yet he demonstrates fierce determination and deep emotional attachment to Hazuki. When Hazuki begins to malfunction and nears the end of his operational lifespan, Kotarō brings him to Kuroda, the creator of the Hybrid Child designs, and pleads for repairs. Kuroda instructs Kotarō to find the mythical "droplet of the moon" to save Hazuki. Kotarō spends a week digging desperately through sand, blistering his hands and refusing to give up despite the seemingly impossible task. The search ends in failure, and Hazuki shuts down. Following this loss, Kotarō undergoes significant maturation. He becomes diligent in his studies, a stark contrast to his previous behavior, and no longer makes excuses to avoid his responsibilities. Approximately one year later, Kuroda contacts him to return a fully restored Hazuki, revealing the "droplet of the moon" was never real and the task was meant as a test or an apology. The relationship between Kotarō and Hazuki is central to Kotarō’s story. Their bond is profound, with Kotarō’s love and care causing Hazuki to evolve further than any other Hybrid Child, developing human-like skin and losing visible joints. This makes Hazuki the antique first model, serial number 0001.