Start in DxD as Motohama: Leveling Up in Another World - Chapter 18: Another Unusual Situation (I)

Han Sheng Li is the Crown Prince of the Great Han kingdom. Since the age of fourteen, he mostly went to the wars with his father, Han Wenji, the great emperor of the Han Kingdom. He's the most favored child of his father but, is the cruelest and ruthless among all of his seven half brothers.

Being poor, weak and introvert, Daniel Lolzwood has always struggled to get a friend, let alone a girlfriend. Who would have thought that one night, a succubus, the most beautiful girl he had ever seen, would come begging him to become her boyfriend? She even gives him a strange system. He can now visit a fantasy world and...Enough with words. I guess it's time to become both rich and handsome while enjoying beauties and revenge..............................Additional tags: [No Yuri] [No NTR] [Adult Content Warning]

This legend tells the tale of the protagonist, Kuroki summoned to protect the demon king from the hero.

Kuroki, unbeknownst to him, suddenly came to another world. He was summoned by the demon king, Modes. The territory which the demon king overrules was in the midst of being destroyed because of the hero. He had no choice but to bow before Kuroki.

“Please save us, Savior Dono!”

Kuroki nodded, deciding to save the demon king due to an unexpected turn of event.

The person whom the world revolves around.