Fairy Tail: I Stole the Ryujin Jakka in My Sleepwalking - Chapter 12: FTRJ -

“You mean that Prince Yu wants to marry me?”

He Xie, a private detective, specializes in investigating extramarital affairs, and once made mistresses and scumbags fearful, he is a good young man with lofty ideals. In the event of an accidental death, you have to start a new life system and complete tasks in exchange for the chance to be reborn.

In “Infernal Affairs”, he worked hard to be a “good person” instead of XXX; in “Youth Bao Qingtian”, he tattooed a sun on his forehead; in “Charlotte Trouble”, Yuan Hua and Qiu Ya sang a plum blossom together, “Lu Xiaofeng” “, “Dragon Babu”, “Kung Fu”, “Bourne Bourne”, “Lord of the Rings”, “Pirates of the Caribbean”…

- Description from novelbuddy

Known as the country’s top onmyoji, Yuuma Kisaragi, who lived to the age of 99, awoke to find himself in an unfamiliar forest. A voice echoed in his mind, revealing that he had been reincarnated into another world at the age of 20.“Good morning, Master. Skip the greetings and please defeat the goblin in front of you.”The voice belonged to a skill acquired during reincarnation called “AI,” designed to assist Yuuma in his new life. When the skill, AI, took over the form of a golden-haired fox yokai, the familiar shape of Yuuma’s shikigami, he decided to embark on a new life alongside AI-chan.The onmyoji and AI-chan’s second life began, tackling various challenges and formidable enemies with the strongest spells and shikigami.

There are a total of seven sins, and a revelation is made to a young man named Alex that he is destined to become a demon lord, more especially a demon lord of sin called LUST.

It's not like he can change that and because he always knew something about himself was different—not in a physical sense, well, maybe a little—and not in a mental sense—but rather because he is thrown into a demonic war that compels him to accept his new reality.

He was only aware of a difference in himself. Relating to him and the ladies who are drawn to him... Well, it was more or less like he was a magnet in front of all the girls, and all the girls were drawn to him like a metal.