Maou Gakuin No Futekigousha - Chapter 40: The naked demon king

In the foreign world, the martial artists dominate the world and the sword of the emperor controls the fate of hundreds of millions of lives! Long Yu, the number one killer on Earth, after dying, takes rebirth in this new world in an aristocratic family of martial artists but faces rejection because he is unable to practice martial arts.

He became a teacher at the best magic academy in the empire.
….For being mistaken for someone else.

Qiao Zhenzhen spent a lifetime, only to later realise that she was the true heroine of a novel. According to the original plot, her life was supposed to be smooth sailing and blissful. But the envious transmigrated second female lead played her cards step by step, and took everything away from her!

After experiencing the divorce of her parents, and miserably failing the college entrance examination, she gets kidnapped along with her little follower of a male lead, Chi Shen. In the end, she came out as the only survivor.

Everyone believed that this was a blessing in disguise, not only was her life saved, but the pervert stalking her for the last three years was also gone! A freak like Chi Shen, who had a weird and eccentric personality, always stank of fish and grease, was better off dead. Everyone said that she ought to be grateful, but none of them knew he died protecting her.
Before he took his last breath, his dying words were-

“Don’t be afraid of me.”

That sentence, mixed with the smell of greasy fish and blood, flashed in her dreams every night for the next decade.

Reborn back into the senior year of her highschool, she walked to the youth in the corner, and in front of everyone’s eyes-

“Don’t hideaway in the dark if you don’t want me to be afraid of you.” The youth pursed his lips, his pale face steadily losing color.

Qiao Zhenzhen smiled: Be nice to me, and I shall allow you to stalk~

- Description from Novelupdates

The story is about a man who’s opposed to the now-common practice of implanting false memories, but is one day given memories of a fictional childhood friend. This fabricated happiness torments him, but then, to his surprise, he meets her – a girl who shouldn’t exist.