She is codenamed 503. She only remembers that she wants to find a place called 'Sky Mirror' and she needs to go back to save someone.
As for where the Sky Mirror is and who the person she wants to save is, she can't remember.
Her only remaining memory told her that the Sky Mirror was an extremely cruel and terrifying place, where many code-named people were born, trained and grew up, and would kill each other when they grew up.
Because in each batch of code-named people, only one can survive, the strongest one.
This story is purely fictitious and any similarity is purely coincidental!
Purpose: Saving people is the first priority