She is classmate of Kei and knows him from middle school upto currently in high school.

Souma was the one who arranged Kei to meet with Misora first time on rooftop of their middle school.

Misora has the ability to 'Save' and 'Reset'. When she says 'Save' the state of everything is at that particular time is stored and later when she says Reset the time of the world will be rearranged to the time and state when she had done the Save. This ability also makes everyone's including the ability user Misora's memory to also be reset thus not knowing of when the last Reset was done.
The only one immune to this ability is Kei due to his ability of retaining and recalling memories.