Natsuki hails from the magical realm of Evergreen, first encountered on Earth in a weakened, rabbit-like form after his energy reserves drain. Discovered by high school student Chihaya Hiiragi, he escapes dissection and forges a pact with her, concealing his true role as a humanoid agent assigned to retrieve hazardous magical seeds. These seeds corrupt humans by magnifying their hidden desires into monstrous transformations.

Chihaya’s nurturing gradually restores his energy, permitting brief shifts into a humanoid state. Physical closeness—particularly kisses—intensifies this recovery, stabilizing his form. As her mentor, he empowers Chihaya to become a magical warrior, their partnership evolving from friction into interdependence. Reinforcements, including his brother Hazuna, later join their mission, sparking both rivalry and teamwork among the agents.

Strategically adept, Natsuki identifies seed-induced disturbances and orchestrates counterattacks while maintaining allegiance to Evergreen’s Queen Tsuyuha through regular reports. Their efforts face threats from antagonists like Fennel, who weaponizes the seeds for chaos, and Calendula, a subordinate consumed by corruption. His dual loyalty to duty and deepening bond with Chihaya tests his adaptability.

Media depictions diverge: the original OVA emphasizes his prickly rapport with Chihaya, while the anime highlights their camaraderie. External challenges include clashes with rivals like Cerulean Blue, who doubts his leadership, and encounters with Emi Kojima, a transfer student whose loyalties shift across adaptations. Energy vulnerabilities and reliance on Chihaya’s support underscore his need for human connection to sustain his mission.