Kei Kusanagi's elder sister.

Spoiler

In a flashback which is shown during a critical "standstill", it is revealed that Kozue was a very bright (though possibly mentally disturbed) girl who felt that human emotions were the cause of much suffering in the world, and sought to escape them by committing suicide, which Kei had had the misfortune of witnessing. The trauma of having done so and the guilt that he had felt at being unable to prevent it led to his first "standstill".