MTL - Harry Potter’s Book of Sin - Chapter 54 Guardian of secrets

When Li Xianyu was 20 years old, he received his father’s inheritance, an 18-year-old great-grandmother who was incredibly beautiful.

Li Xianyu: “Great-Grandma, why are you so young?”

Great-Grandma: “That’s because I died when I was 18 years old.”

- Description from Novelupdates

…but it seems pretty doable.

He was a Martial Arts prodigy, a master of using poisons!He descended the mountain to search for his junior sister but unexpectedly became the School Beauty's bodyguard… From then on, a demon-like student was born! He pursued the School Beauty, took down bullies, and was revered as the supreme ruler of the city! He cultivated Ancient Martial Arts, annihilated formidable enemies, and swept across all directions to ascend to the Peak.

“Impossible… Why are you not following my orders!? Clearly I sucked your blood, why!?”

On the very first day of high school, the youth subscribing to the philosophy of “absolutely ordinary everyday life,” Kujou Hisui, encountered disaster head on: He was attacked by Rushella Daamu Draculea, the vampire who had lost her memory. Rushella was greatly intrigued to find her vampire powers ineffective against Hisui. Not only did she make Hisui’s home her own, she even enrolled in his school to stick to him–

Hisui had no choice but to help Rushella regain her memory. But before one crisis could be resolved, many more had risen. With tempting advances from the mysterious Sudou Mei, and kidnapped by the girl from the Supernatural Investigations Section in search of Rushella, Hisui’s ordinary life has been replaced by its complete opposite, drama–

Romantic comedy and supernatural action in a school setting!