Haruki, Misora

春埼美空

She is Kei's classmate and has known him from middle school through to their current high school years.
Souma was the one who arranged for Kei to meet Misora for the first time on the rooftop of their middle school.
Misora possesses the ability to 'Save' and 'Reset'. When she says 'Save', the state of everything at that specific moment is stored; later, when she says 'Reset', the world's time is rearranged to the time and state of the Save. This ability also resets everyone's memories, including Misora's own, meaning no one knows when the last Reset occurred.
The only person immune to this ability is Kei, due to his power to retain and recall memories.

Spoiler