Rachel no Omoide

The protagonist is Kaori Kashima, a second-year high school student. She and her best friend Rachel are on their way home when suddenly, a person in a black hood appears and fatally stabs Rachel. Somehow, Kaori acquires a time machine and manages to go back in time. But she soon realizes something's different here—it's a parallel world. Rachel is alive and well, but Kaori feels she must return to the original timeline. Where and when did this parallel world split off? Searching her memories for answers, she attempts time travel again at the moment of divergence, but—