Martial King's Retired Life - Chapter 94Book 15:

Traveling through the world of One Piece, Ron, who became Kuina’s fiancé, awakened the god-level theft system.

In order to become stronger and change the fate of Kuina’s death, and to survive in this dangerous era of great pirates, Ron started to steal Sauron’s top kendo talent.

And that’s just the beginning.

In order to become stronger, Ron embarked on a journey to steal the talents of the world’s strongest.

“Ding! Congratulations to the host for stealing Karp’s physical talent!”

“Ding! Congratulations to the host for stealing Nika’s talent!”

“Ding! Congratulations to the host for stealing Shanks’ domineering talent!”

“Ding! Congratulations to the host…”

- Description from novelbuddy

After coming out of a tunnel… it was pitch dark.

Ding![The Host has died!]A system message popped up in front of Nell's body after his last dying breath.Ding![Dark Evolution has been activated to absorb the death essence of the host.]And then he opened his eyes to find himself in the past right before the battle started.'Ah! Am I back to the past?'Ding![Please choose a partner to start the Dark Evolution process.][Partner Condition: Must be a living creature from the Monster category, not the Human category][Partner Gender has to be Female.]'Partner? A monster?'Nell's heart pounded in his chest, and he felt a cold sweat coming on.'A female monster?'

Once through the pirates, Lu En became the son of Monkey d Dragon, a grandfather named Monkey d Karp, and an unreliable and neurotic younger brother named Luffy.

Fortunately, the ghost back system is activated!

The completion of the ghost back is 10%, and the training effect is increased by 100 times!

The completion of the ghost back is 20%, and the training effect is increased by 1000 times!

…

One day, Lu En, a ghost-backed master, appeared in front of Luffy who was shouting to go to sea, and made Luffy cry on the spot.

Luffy: “The pirates in the sea are all monsters like my brother? I don’t want to go out to sea!”

Garp: “For a long time, the two boys, Long and Luffy, with small arms and calves, I really doubt that they are not my sons and grandsons. Like Lu En, they are the grandsons of the old man!”

Warring States: “The strongest creature in the sea, land and air is Kaido? No, it’s Lu En!”

- Description from novelbuddy