Mokudan is a woodcutter living in the mountains of Japan during the Heian period. He is a kind-hearted man who has struggled with poverty, and his deepest desire is to own a small piece of land and live a peaceful life. Despite his humble circumstances, he possesses a strong sense of altruism. When the demon Nue begins terrorizing his village, Mokudan is moved by the suffering of the villagers and decides to confront the creature. His bravery and selflessness are rewarded when the gods choose him to be their earthly representative. They entrust him with a precious elixir that grants immortality to whoever drinks it, instructing him to safeguard it and pass it down through future generations until a worthy person eventually consumes it. Mokudan accepts this duty without hesitation, and from that point his life becomes deeply intertwined with the Phoenix, a mystical bird that symbolizes immortality and rebirth.

Mokudan is defined by his introspective nature, idealism, and emotional sensitivity. He is deeply guided by his values and moral convictions, caring profoundly about those around him. His primary motivation shifts from the simple wish for a tranquil existence to a solemn commitment to protect and transmit the elixir across time. Within the story, he serves as a foundational figure in the Karma chapter, acting as the initial human custodian of the immortal gift. His descendants appear in later episodes, and his legacy of courage and integrity echoes through their interactions with the Phoenix.

Notable abilities are not supernatural; Mokudan's strength lies in his moral fortitude, bravery, and unwavering sense of duty. He evolves from an ordinary peasant burdened by hardship into a guardian of cosmic significance, embodying the themes of altruism and resilience that run throughout the series. His key relationship is with the Phoenix itself, which links him to the broader cycle of life and death explored in the narrative.