Fushi is the central protagonist of his narrative, an initially nameless and consciousness-less immortal being created by a mysterious entity known as the Beholder. Sent to an empty, arctic world, Fushi begins as a simple white orb, or a rock, and then a patch of moss, copying the forms of things that provide physical stimulation. The being's first meaningful transformation occurs when a wounded, lone wolf dies near it, and Fushi takes on the animal's form. In this shape, the wolf encounters a young, unnamed boy living alone in an abandoned village. The boy, mistaking the wolf for his deceased pet Johan, cares for the creature. After the boy succumbs to a severe injury, Fushi, driven by the boy's dying wish to never be forgotten, permanently copies his human form, establishing the white-haired, youthful appearance that becomes his primary identity.

Fushi’s personality is defined by its origin as a blank slate. In the beginning, it possesses no emotions, identity, or understanding of the world, acting only on instinct. As it travels and meets new people, particularly the young girl March who gives it the name Fushi, meaning "immortal," it begins to develop a sense of self and a range of complex human emotions. Fushi is initially confused by concepts like pain, hunger, and social interaction, often displaying brutal honesty due to its lack of social nuance. However, the repeated experience of forming deep bonds with humans only to lose them to death or tragedy profoundly shapes its character. This cycle of connection and loss instills in Fushi a deep sense of survivor guilt and a growing weariness of its own immortality. It frequently questions the purpose of living forever while watching those it loves grow old and die, leading to periods of self-imposed isolation to avoid further heartbreak. Despite this pain, Fushi’s core motivation becomes the desire to honor the memories of its lost companions by protecting others and, eventually, finding a way to create a peaceful world.

Within the story, Fushi serves as a vessel through which the narrative explores themes of humanity, identity, and the nature of life and death. Its journey is not one of a traditional hero but of a being learning what it means to exist. Key relationships are the primary catalysts for its development. March, a young girl who was to be a sacrifice, becomes a maternal figure who gives Fushi its name and teaches it about family. The aging criminal Pioran acts as a mentor, guiding Fushi through social norms and the practicalities of living. Gugu, a boy with a masked face, becomes a brotherly figure, teaching Fushi about love, jealousy, and sacrifice. The warrior Hayase develops a dangerous, obsessive love for Fushi, leading to a centuries-long legacy where her descendants, known as the Guardians, both protect and pursue Fushi with the same twisted devotion. Others, like the aspiring writer Tonari and Prince Bon, provide crucial companionship and strategic aid in Fushi’s battles against the Nokkers, the primary antagonistic force that seeks to destroy Fushi as an aberration of nature.

Throughout its long existence spanning centuries, Fushi undergoes profound development. It evolves from a simple, reactive orb into a deeply emotional and morally complex individual. A significant turning point is its discovery of the ability to resurrect the dead, a power that brings beloved friends back to life but also carries immense consequences. As Fushi ages, it grows from seeking revenge to striving for understanding and peace, eventually spreading its body across the entire planet to suppress the Nokkers for six hundred years. It learns to navigate complex political landscapes, command armies, and ultimately grapple with philosophical questions about coexistence with its enemies when they evolve to live within humanity.

Fushi possesses several notable abilities. Its primary power is immortality coupled with near-instantaneous regeneration, allowing it to recover from any fatal wound. It is a shapeshifter, capable of transforming into any being or object that has left a strong impression, which can be caused by physical pain, emotional impact, or proximity to death. These forms, or "vessels," retain all the physical capabilities of the original, such as the strength of a bear or the agility of a wolf. Fushi can also create objects—such as weapons, clothing, or food—from its own body, but this ability is limited to solids that have previously caused it harm or left a significant impression. Later in its journey, Fushi learns the ability to resurrect living beings, but this process requires the soul to still be present and can be mentally and physically draining. Furthermore, Fushi possesses a form of empathy, feeling the physical pain of those nearby and thus detecting distress in others.