Hailing from the magical realm of Evergreen as an agent of Queen Tsuyuha, Natsuki arrives on Earth to recover stolen magical seeds. Depleted magical energy traps him in an animal form resembling a rabbit or ferret, resulting in his capture and near-dissection by a school nurse. Chihaya Hiiragi rescues him, offering shelter despite restaurant rules forbidding pets.

His animal state persists until physical affection from Chihaya replenishes his magic. A bathing scene where Chihaya cares for him culminates in a kiss, accelerating his rapid maturation into human form, surprising Chihaya and forging their partnership. As a human, he detects seed disturbances and mentors Chihaya to become the magical warrior Carmein, advising her during battles against seed-created monsters. Telepathically communicating with Chihaya consumes significant energy, limiting its use.

Natsuki's background identifies him as Hazuna's brother; both are tasked with seed recovery. His mission complicates when confronting seed thief Bergamot—Chihaya's father—and the primary antagonist Fennel, who manipulates events. Natsuki's reliance on Chihaya's affection to sustain his power recurs throughout the series, especially when weakened. Sleeping in her room replenishes energy but risks exposure, sometimes forcing a temporary reversion to animal form.

During the *Summer Special* OVA beach camp, classmate Nami Minami develops unspoken feelings for Natsuki. Her unresolved desires attract a seed that corrupts her, linking her to the "Netherworld Tree" and triggering a new crisis. Natsuki participates in resolving this incident as part of the ensemble effort, with limited direct interaction with Nami. Across all media, he consistently mentors Chihaya while navigating threats from Evergreen fugitives and seed-induced monsters.