Kagari, often referred to as the Key, is a central figure whose existence is tied to the fate of the planet. She is not human but a terminal created by the Earth's will, born from the energy known as Aurora to observe and interact with humanity. Her primary function is to assess the state of the world. When the planet's energy is depleted or its balance is lost, she can initiate a process called salvation or relief, a form of destruction that resets the biosphere, which humanity perceives as an apocalypse.

Physically, Kagari appears as a young, pale-haired girl with short silver-white hair and deep lavender eyes. She is typically depicted wearing a simple black dress with white ruffles, and her most distinctive feature is the pair of long red ribbons tied around her wrists. These ribbons are not merely decorative; they are weapons made of Aurora that can extend, multiply, deflect attacks, and strike with tremendous force to protect her.

Her personality varies significantly across different narratives and routes within the story, largely depending on her level of interaction with humans and her stage of development. In her initial form on Earth, Kagari rarely expresses emotion or speaks, appearing as a pure and detached observer. She can display flashes of a darker personality, reacting with anger to perceived perversions or even smiling sinisterly at the suffering of others, indicating a perspective that is utterly alien to human morality. As she learns more about humanity, particularly in the Terra route, she becomes more intelligent and talkative, though her understanding remains flawed. She can be rude, insulting humanity for its constant meaningless conflicts, and proposes bizarre, impractical solutions to human problems, leading her to be described as a clueless princess rather than a divine being.

An important distinction exists between the Earth Kagari and the Moon Kagari, who are effectively twin sisters born from the same energy when the Moon split from the Earth. The Moon Kagari is the sole inhabitant of the lunar surface, living in utter solitude while using a vast theorem to simulate and search for a future where life can continue. She is initially far more hostile and distant than her Earth counterpart, speaking in a compressed language whose sound can harm humans, and she reacts violently when approached, even killing the protagonist multiple times. However, she can gradually warm up through persistent, kind interactions. Her driving motivation is a sense of resentment and loneliness; she envies the Earth for being full of life while she is trapped on a cold, barren rock, though her ultimate goal is to save life on the planet below.

The protagonist, Kotarou Tennouji, shares the most significant relationship with Kagari. Their fates are intertwined when he is mortally wounded by her, and Kotori uses a piece of Kagari's ribbon to save his life, granting him his Aurora abilities. In the Moon route, Kotarou proves to be the key to helping the Moon Kagari complete her calculations, and they form an unlikely bond. In the Terra route, he works directly with her to find good memories for the planet in a desperate attempt to avert salvation. Kotori, as a Druid, also has a crucial role as the one who protects Kagari.

Kagari possesses several notable abilities beyond her ribbon weapons. She has a passive power that makes her invisible and imperceptible to most living beings, allowing her to observe battles or events without being noticed. Her singing, known as the Song of Destruction, is a strange melody that signals the beginning of salvation. She can also use compressed language, speaking in a single, incomprehensible character that contains a vast amount of information. Over the course of the story, Kagari develops from an almost mindless force of nature into a being capable of understanding, frustration, and even affection, transitioning from a symbol of the planet's judgment to a partner in a desperate search for a hopeful future.