Gekitotsu no Hexennacht - Volume 4, Chapter 4: If Some Return

A teenager named Evan Mason was reincarnated into the magical world of Harry Potter and goes to school at Hogwarts!
With only his knowledge of the future and great talent for magic, he takes the path towards the top of the magic world, only to realize that ever step he took, changed everything!

- Description from Novelupdates

I found a girl of unknown origin outside; she claims to be a time traveler from a post-apocalyptic era.Great calamity is imminent, with abominations spreading across the land, human civilization annihilated, and even the world itself shattering, all beyond redemption.As for myself,I am considering how to confine this beautiful girl, who has temporarily lost her powers, within my home.

From a media tycoon’s daughter to a poor plumber’s girl, but it seems that Yu Yao is unfazed? She opened the eyes of the sky, and her life has been opened all the way. She is an eighteen-year-old girl, but as soon as she turns around, she becomes a big boss who no one in the entertainment industry dares to offend.

- Description from Novelupdates

Lin Xiao was an orphan who strived to get stronger while living with his sickly younger brother in an unnamed small village.