Luna is a central figure in the Casshern franchise, appearing in multiple incarnations with distinct roles. In the 2008 anime Casshern Sins, she is a mysterious and almost divine being known as the Sun that was Named Moon, a title that reflects her paradoxical nature. She possesses immense power to heal and grant life, making her the wellspring of existence for the robotic world and earning her the title Source of Life. She was summoned by humanity as a savior, a figure of hope who could bring salvation from the oppressive rule of the robot leader Braiking Boss. However, this very power made her a target, and she was assassinated by Casshern on the orders of Braiking Boss. Her death triggered a cataclysmic event known as the Ruin, which plunged the world into decay and suffering. In this version, Luna is both a symbol of salvation and the cause of despair; her existence and fate are inextricably tied to Casshern, and her presence lingers as an object of search and desire for those seeking redemption or an end to their pain. Her personality is enigmatic and aloof, embodying both benevolence and a dispassionate detachment that stems from her transcendent nature. She does not shy away from the concept of death, and in some interpretations, she is seen as the one who can grant it as a release from eternal suffering. Key relationships include her connection to Casshern, whose act of killing her defines his own journey and the world's fate, and her protectors, such as the guard Dune. Throughout the series, Luna undergoes a form of development as the characters who seek her discover that her salvation may not be what they expected, leading to a deeper understanding of life and death.

In the original 1973 anime Neo-Human Casshern, Luna appears as Luna Kozuki, a human girl who fights alongside the hero Casshern against the robot army of Braiking Boss. In this version, she is the daughter of a scientist and uses a powerful handheld weapon called the MF Gun to battle the robotic enemies. She is a capable and brave ally, and her relationship with Casshern is one of friendship and mutual support, forming a key part of the resistance against the mechanized tyranny. Her role is more action-oriented and grounded, serving as a human counterpart to the android hero.

Across all versions, Luna is defined by her deep connection to the central themes of the Casshern story: life, death, salvation, and the cost of immortality. Her abilities range from healing and granting life in the Sins incarnation to combat proficiency in the original series, but in every case, she is a pivotal character whose actions and very existence drive the narrative.