Characters that have changed

Well you already know what's gonna happened..
Ok maybe a little bit...
...
The girl with black hair reseted the whole world
No one seems to understand me
It seems like everyone just forget about me
Or is it just that the girl reseted the time, but I still have the memories of the last timeline
Every time I see the girl with black hair, she resets the timeline.
Why?
What was her propose, and what dose she need?