Superpower: Become a God of Fighting from the Beginning of Practice

The story is set as a double time travel. The male protagonist traveled to a world where supernatural powers and monsters were rampant, picked up a notebook, and traveled again. Back 80 years ago, a year after monsters first invaded the human homeland, a girl was rescued in the last bastion city of mankind. Her sister was known as the strongest human in the school. This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.