One Piece: I'm Destined to Be the Son-in-Law King! - Chapter 21: Sudden Pirate Incident

After a mishap, he accidentally became the protagonist. Unexpectedly, he and the villain cultivated together and they actually became dao companions.

“If you want to be with me forever, there is only the supreme road of achievement. Are you willing to travel with me?”

“Of course I’m willing.”

……

In Li JinZhi’s view, the protagonist and villain’s coexistence should be like swords and spears facing each other, a feud.

But when he crossed into the novel and became the protagonist; when he placed all his trust in the man who was supposed to be the villain; when the other side revealed some unusual feelings to him; when he held Ye Yu’s hand to the altar, he discovered that the relationship between the protagonist and villain isn’t as dull as he imagined.

When the monotonous pattern is broken, the feelings of resentment turns into sharing a common destiny.

- Description from Novelupdates

The notorious imperial court lackey and the meritorious military general were well-known for their mutual dislike, a pair of arch-enemies set in stone. Who could have foreseen the unexpected change in the winds of heaven: the general injured on the battlefield, both legs disabled, and even forced by the emperor to marry his nemesis.With one marriage edict, two people with mutual hostility are forced to marry and live under the same roof. Yet after these long days together, they come to the realization that this fellow is not truly devoid of any saving grace/unattainable and high out of reach.

King Grey has unrivaled strength, wealth, and prestige in a world governed by martial ability. However, solitude lingers closely behind those with great power. Beneath the glamorous exterior of a powerful king lurks the shell of man, devoid of purpose and will. 

Reincarnated into a new world filled with magic and monsters, the king has a second chance to relive his life. Correcting the mistakes of his past will not be his only challenge, however. Underneath the peace and prosperity of the new world is an undercurrent threatening to destroy everything he has worked for, questioning his role and reason for being born again. 

Jin Runcandel is the youngest son of the greatest Swordmaster. After he gets himself kicked out of their clan, he meets a tragic fate…but before he dies, a God gives him a second chance. Now he must use his new power for good.