Fushi is the central character of the narrative, an immortal being initially created and sent to Earth by a mysterious entity known only as the Beholder. At the start of his existence, Fushi is presented as a blank slate, a white orb with no emotions, identity, or consciousness. His primary purpose, as defined by his creator, is to survive, grow, and ultimately preserve the world.

Fushi possesses the unique ability to transform into any object or living creature that leaves a strong impression on him. This can be triggered by powerful physical stimuli, such as being injured, or by profound emotional connections. As he wanders the earth, he begins his journey by taking the forms of a rock, moss, and then a mortally wounded wolf. After encountering a lone boy living in an abandoned arctic village, Fushi copies the boy's appearance in response to his final wish to be remembered, adopting this human form as his primary default shape.

In terms of personality, Fushi begins as a completely neutral and instinctual entity. He lacks a sense of self, social norms, and the ability to communicate or understand complex human concepts. A key aspect of his personality development is his brutal honesty, as he does not understand the concept of mincing words or telling lies. As he learns, he is initially confused by basic human functions like eating, but quickly develops a voracious appetite. Over time, as he forms bonds with people, his personality becomes more complex, allowing him to feel a wide range of emotions including joy, confusion, anger, and profound grief.

The core of Fushi's motivation changes drastically throughout the story. Initially, his only driving force is the instinct to move forward and transform. After taking the boy's form, he travels south out of a sense of obligation to fulfill the boy's unspoken wish to see new lands. His most significant early motivation becomes the desire to protect and be with those who show him kindness, such as the young girl March who gives him his name. However, as he witnesses the deaths of his friends due to his inability to fully understand or prevent human violence, his motivations become more complex. He is driven by a deep desire to prevent the pain of loss, eventually seeking to become strong enough to protect everyone and to find a way to give them eternal life, which leads him into centuries-long plans to eradicate the antagonistic Nokkers.

Fushi's role in the story is that of a witness and a catalyst for change. He is an immortal observer of humanity, experiencing the world through the people he meets. His very presence alters the lives of those around him, for better or worse, as they are drawn to his unique nature. As he learns from humans, he also acts as a vessel for their memories and forms, carrying their legacies forward long after they are gone. His primary antagonists are the Nokkers, a mysterious enemy force that seeks to steal his forms and memories, threatening his sense of self and his mission.

Key relationships are the primary drivers of Fushi's development. The Unnamed Boy, whose form he takes, gives him his first identity and a sense of purpose. March, a young girl from a tribal village, names him Fushi which means immortal, and their relationship establishes a protective, familial bond, with her acting as a maternal figure. His journey with the elderly former prisoner Pioran is crucial, as she teaches him how to speak, read, write, and function in human society, essentially raising him. Gugu, a boy with a disfigured face, becomes his first true brother-like friend, teaching him about family, hard work, and the nature of love. Tonari, a girl on a prison island, challenges him to open his heart again after loss and teaches him about friendship and trust. A more complicated and obsessive relationship exists with Hayase and her descendants, who develop a dangerous fixation on Fushi, often causing him great pain and suffering.

The character's development is the central focus of the narrative. Fushi's journey is a slow and painful progression from an object to a human being. He begins with no consciousness, then learns to feel physical pain, then simple emotions like joy and sadness. His development is not linear; each loss of a loved one sets him back, causing him to harden his heart and isolate himself from others for long periods. Over centuries, he learns complex emotions such as survivor's guilt, the burden of immortality, and the desire for death. He eventually evolves from a being who merely reacts to stimuli into a proactive, strategic protector capable of complex planning and wielding immense power to reshape the world.

Fushi possesses a range of notable abilities tied to his immortality. His primary ability is Shapeshifting, allowing him to transform into any being whose form he has acquired. Another key ability is Object Creation, where he can manifest any non-living object that has left an impression on him, typically by causing him pain, such as a spear or a knife. He has a potent but costly ability to heal others and even resurrect the recently deceased by transforming a part of his body into their form. This process is incredibly draining and requires a strong connection to the soul of the person he is reviving. Fushi is completely immortal and possesses a limitless healing factor. Even if his body is completely destroyed, he will eventually regenerate. However, he still feels the full pain of any injury, and a powerful enemy like the Nokkers can temporarily drain or steal the forms he has acquired.