Milo Nekoyanagi, nicknamed "Panda-sensei" for the distinctive black bruise encircling his left eye, runs a clinic in Imihama. Possessing a natural medical genius, he focuses his skills on developing medicines, including formulations derived from illegal mushrooms, in a desperate quest to cure his sister Pawoo's severe rust infection. His core motivation is a deep desire to help others, especially Pawoo.

His path intersects with fugitive Bisco Akaboshi when Bisco seeks help synthesizing a healing mushroom for his mentor, Jabi. Following Jabi's suggestion, Milo agrees to accompany Bisco on a journey to find the Rust Eater mushroom, hoping it can cure Pawoo. He leaves his sister with medicinal supplies and departs Imihama with Bisco. Traveling across rust-ravaged wastelands, Milo consistently demonstrates compassion, healing Tirol Ōchagama by removing a balloon worm and correctly diagnosing/treating a non-rust skin condition affecting children in a settlement, subsequently negotiating Bisco's release.

His medical expertise proves vital in deciphering the Rust Eater's properties. Upon discovering it requires activation via contact with Mushroom Keeper blood—specifically Bisco's—he formulates a functional cure. This breakthrough heals Pawoo but draws the antagonistic Governor Kurokawa, who seeks to monopolize the cure. Facing duress, Milo's typically gentle and empathetic personality reveals combat pragmatism; he attempts to poison Bisco with sleeping mushrooms to prevent a suicidal rescue mission for Pawoo and Jabi, acknowledging Bisco's worsening rust infection and physical deterioration.

Milo's loyalty to Bisco intensifies significantly. Captured by Kurokawa and implanted with a puppetshroom for control, he endures torture rather than reveal the Rust Eater's secret. He ultimately discloses the blood-activation requirement to save Bisco during Kurokawa's forced confrontation. Later, sustaining life-threatening injuries from rust arrows protecting Bisco during their escape, Milo recovers after emergency treatment by Bisco. He then intervenes in the final battle against Kurokawa, saving Bisco by destroying the governor's prosthetic arm. After Bisco sacrifices himself to defeat Kurokawa, Milo accepts his partner's death, carrying Bisco's spirit within him and fulfilling their promise that their souls would remain connected.

Milo modifies his archery techniques to compensate for lesser physical prowess compared to Bisco, enhancing arrows with explosives and chemical agents. He shares a bond with Bisco's giant crab companion, Actagawa, initially struggling to gain its trust before eventually succeeding. His foundational relationship with his sister Pawoo continues to motivate his actions even as his partnership with Bisco deepens into a profound mutual commitment.