Hayashi Fuyou is a posthumous character whose memory serves as a foundational element for one of the series' recurring robotic characters. She was the sickly daughter of the brilliant but misguided scientist Hayashi Ryuuzan and died of tuberculosis before the main events of the story. Her physical appearance was that of a thin, very pale girl with pink hair and brown eyes, reflecting her long struggle with illness.

Fuyou's personality is portrayed through flashbacks and the actions of those who knew her. She is depicted as a gentle and lonely child, isolated by her poor health. Despite her physical weakness, she demonstrated a degree of selflessness, or perhaps a desperate desire to please her father, by agreeing to his dangerous experiments to extract her own personality data, an act that ultimately hastened her death. This tragic passivity and her longing for simple joys, like wanting to swim in a lake, define her short life.

Motivated by a combination of loneliness and a wish to connect with others, Fuyou formed a pivotal friendship with Ebina, an amanto who was also alone. Meeting him by a lakeside, she confessed her wish to swim with him one day, a dream her lung disease prevented her from realizing. In response, Ebina promised to protect that lake until she was cured, a vow he keeps long after her death. This friendship and promise are central to her legacy and the motivations of other characters.

Fuyou's role in the story is almost entirely as a catalyst for other events. While she never appears alive, her existence is the emotional core of the Fuyo Arc, the storyline that introduces the robot Tama. Professor Ryuuzan, unable to cope with the loss of his wife and the impending death of his daughter, began creating toys to comfort Fuyou. These toys eventually evolved into highly advanced androids, with Tama being the most significant. The personality data extracted from Fuyou was used as the foundation for Tama's consciousness, making the robot a sort of spiritual successor and living memorial to the deceased girl. The conflict of the arc revolves around protecting this data.

Her key relationships are few but deeply impactful. Her father, Hayashi Ryuuzan, is driven by grief to create artificial life in her image, blurring the lines between love and obsession. Her friendship with the amanto Ebina is a pure and simple connection that transcends species and death, as Ebina remains fiercely loyal to her memory and the promise he made. Most importantly, her bond with Tama is not a direct relationship but an inherited one; the robot maid carries Fuyou's personality, memories, and gentle spirit within her, effectively giving Fuyou a second, artificial life.

Fuyou herself undergoes no development within the narrative, as she is already deceased. However, her influence on the development of others is immense. Her death is the single event that sets Ryuuzan on his destructive path and is the reason for the creation of Tama. As a character, she represents the pain of loss and the desire to preserve the memory of a loved one, themes that are common in the series. Her notable abilities are not physical or combative. Instead, her most significant attribute is the strength of her personal data, which is stable and complete enough to serve as a complete personality matrix for an advanced android like Tama, a feat that seems to be unique within the story's world.