Lei Xiaoyu, alias Rei Shaoyū, is a 19-year-old ex-operative of the Heilei assassin guild now employed as a bodyguard for the Shinoyama Clan. Charged with safeguarding Sayo Shinoyama during her stay with an imposter posing as his half-uncle Polka Shinoyama, he balances protective duties with covert surveillance of the usurper. Molded into a contract killer by the Lei Family from childhood, Xiaoyu undertook his first assassination at nine or ten, targeting terrorists threatening his kin. The mission brought him face-to-face with Majiri, Kana, and Momoya Agakura, who had slain the targets first. When Xiaoyu attacked Majiri, she swiftly severed his limbs in a non-fatal confrontation, proposing their return in exchange for his allegiance to the Agakura Clan. Rescued by the Heilei, he later discovered his family’s plan to infect him with poisonous bacteria as a host. Rozan Shinoyama intervened, purchasing Xiaoyu and equipping him with biomechanical prosthetics—limbs indistinguishable from organic tissue, including a right arm forged from the lightning dragon Urdwigia. These enhancements feature retractable defensive claws, concealed eavesdropping microphones, and combat-ready durability. Xiaoyu’s fanatical loyalty to Rozan, whom he idolizes as a father, clashes with his contempt for Rozan’s biological son Polka, deemed unworthy of inheritance. This rivalry spiraled into obsessive jealousy, convincing Xiaoyu he alone deserves recognition as Rozan’s heir—a conviction fueling his resolve to eradicate both Polka and the imposter inhabiting Polka’s identity. Following Polka’s disappearance and the imposter’s arrival, Xiaoyu guards Sayo while scrutinizing the impostor’s every move. Though Rozan trusts the double, Xiaoyu covertly monitors communications and pursues independent investigations, documenting interactions at the imposter’s residence for the Shinoyamas. His demeanor masks calculated ambition behind a façade of humility and eagerness to serve. Unyielding in beliefs, he dismisses contradictions to his suspicions, channeling devotion into relentless initiative—tracking investigators, gathering intel, and preemptively neutralizing perceived threats. Youthful in appearance with short black hair and delicate features, Xiaoyu dresses in long sleeves, striped shorts, sock garters, and gloves hiding his synthetic fingers. The prosthetics’ seamless integration enables combat agility without betraying their mechanical nature. Rooted in Heilei conditioning and the trauma of limb loss, his worldview equates worth with utility and unwavering loyalty. The surname “Lei” (thunder) echoes both his lightning-infused arm and the Heilei’s “black lightning” symbolism. While confident in his prowess, allies of the imposter dismiss him as a minor threat compared to supernatural foes—a miscalibration of risk underscoring his human-driven vendettas.