Kagari

A young, high school age-looking girl.

She is referred as the "key" throughout the game. The last two concluding arcs of the game called "Moon route" and "Terra route" revolve around her.

Kagari, the main heroine of Rewrite, has the power to destroy all intelligent life on Earth, though can also initiate a period of re-evolution, which restarts the process of evolving another means of intelligent life. However, this is done with the use of the Earth's energy, and by the time the events of Rewrite occur, there is no more energy left to do another re-evolution. Gaia wants to capture Kagari to ensure the destruction of humanity, but Guardian wishes to seek out Kagari to kill her, ensuring that human life continues.
(Source: Kanon-finale)