Yakumo Fujii begins as an ordinary Tokyo student working part-time. His life shifts when he meets Pai Ayanokoji, revealing his late father's promise to help her, the last Sanjiyan Unkara, become human. Yakumo reluctantly assumes this quest. During a chaotic incident, Pai's demon companion Takuhi fatally wounds him. To save him, Pai merges his soul with hers using her Sanjiyan powers, transforming him into her immortal Wu. This bond grants Yakumo near-instantaneous regeneration from any injury and immunity to death as long as Pai survives, though he still feels pain. The Wu symbol (the character for "void" or "nothingness") manifests on his forehead, marking his new existence.

Initially driven by a desire to regain his mortality by fulfilling the promise to make Pai human, Yakumo's motivation evolves. He transitions from reluctance to unwavering dedication, fueled by a deepening bond with Pai. His early role involves protecting her from demons and monsters seeking her power or the Statue of Humanity artifact. Despite initial vulnerability, his immortality forces relentless confrontations, forging resilience. His protective mission expands against threats like the demon Benares and the followers of the sealed demon god Kaiyanwang, who seek their master's resurrection.

In the OVA "Seima Densetsu," Yakumo locates Pai in Tibet after a four-year search following her disappearance, finding her amnesiac. He guides her to a holy land to restore her memories, battling monstrous adversaries. This arc showcases his increased combat skill and strategic thinking, moving beyond reliance on regeneration. He actively utilizes summoned creatures (beastmasters) and martial abilities, reflecting training during his search. His relationship with Pai deepens through patience during her memory loss and moments where their protector-protected dynamic reverses.

Later narratives like the sequel manga "Genju no Mori no Sonansha" and "Kiseki no Yami no Keiyakusha" depict Yakumo facing escalating threats. He masters advanced beastmasters and demonic powers. His immortality enables unique tactics, enduring catastrophic damage to exploit enemy weaknesses. His central motivation remains Pai's safety and well-being, their bond solidifying into mutual reliance. Romantic feelings develop, becoming a core driver for his actions. He interacts with allies like the profit-driven Ling Ling and the beastmaster-wielding Haan, but Pai remains his primary emotional anchor. External conflicts, such as Pai's arranged marriage to another Sanjiyan, Koko, test yet reinforce his commitment.

Yakumo's core ability is his Wu immortality: rapid regeneration, pain tolerance, and survival of dismemberment or disintegration, contingent on Pai's survival. Post-timeskip, he augments this with learned skills including summoning and controlling energy-drawing beastmasters and martial combat. His physical resilience makes him an implacable foe, often overcoming stronger entities through endurance. Key traits include unwavering determination, loyalty to Pai, and a protective instinct extending to innocents. His personality balances inherent kindness with hardened resolve from centuries of conflict, retaining a core desire for normalcy despite embracing his role.