Luna Natsuki is a mysterious and almost divine figure from the anime Casshern Sins, known as the sun that was named moon. She is a young girl with long gray hair and blue eyes, often depicted wearing a hat. She wields a fantastic power to heal and grant life, which made her the pivotal point of hope and despair in the world. She is widely viewed as the symbol of salvation, immortality, and the tragic price of escaping death. Her existence and fate are deeply tied to Casshern, as she is the source of life for the robotic world.

In terms of background, Luna was summoned by the human race for salvation, becoming known as the source of life, a living embodiment for the world. She lived beloved by both robots and humans, giving life equally to all. However, Braiking Boss, the leader of the robots who had subjugated humanity, saw her as a potential threat to his rule. He ordered his three most powerful cyborg warriors, Casshern, Dio, and Leda, to eliminate her. Casshern, the strongest, tracked her down and killed her by piercing her chest. This act triggered the ruin, a cataclysmic event that caused the world to deteriorate. The robots lost their ability to live indefinitely and began rusting and dying, while the atmosphere became filled with poison. Luna disappeared after being killed by Casshern, but she appears to those who seek salvation, including robots and humans, throughout the story.

Luna's personality is enigmatic and ethereal. She is gentle and compassionate, granting life and hope to those who are suffering. However, her power also brings despair, as her death unleashed death and decay upon the world. She is a figure of immense tragedy, as her gift of life becomes a curse that leads to the end of the world. Her motivations are rooted in her role as the giver of life, but the series explores the duality of her existence: she is both a savior and a source of doom. Her character embodies the themes of life, death, and the consequences of seeking immortality.

In terms of role in the story, Luna is the hidden main antagonist and the central mystery of the series. Her death sets the entire plot in motion, and her memory drives Casshern's quest for truth and redemption. She is a symbol of hope for some and a source of fear for others. Her reappearances or manifestations guide the narrative, often appearing to those in need of salvation or facing despair. She is the ultimate goal for many characters seeking eternal life or relief from the ruin.

Key relationships include Casshern, who killed her and is haunted by the act. Dio, another cyborg warrior sent to kill her, has a deep obsession with her and seeks to understand her death. Leda also participated in the hunt. The robots and humans who remember her view her with reverence or desperation. Luna's relationship with Casshern is the most crucial, as his actions define her legacy and his journey is about reconciling with his sin of taking her life.

Development-wise, Luna is largely a static figure in the past, but her influence evolves through the perceptions of other characters. The series explores the consequences of her death and the search for her true nature. In flashbacks, she is shown as innocent and kind, but as the story progresses, her role as an antagonist emerges, as her existence challenges the natural order of life and death.

Luna's notable ability is her power to heal and grant life. She can give life to robots and humans, making her the source of vitality for the world. This power also brings death when it is removed, as seen in the ruin following her death. She is also able to appear to those who seek her, though her true nature remains ambiguous. Her powers make her a target for those who want to exploit her for eternal life, but they also make her a symbol of the tragic price of escaping death.