One Piece: I'm a Marine Admiral, and My Brother Is Luffy

Monty D. Xiu En transmigrates into the world of pirates and becomes Luffy's chronicle-loving older brother.

From the start, he awakens the [Twin Cause-and-Effect System].

Whenever Luffy causes trouble, Xiu En earns rewards; meanwhile, the medals for Xiu En's achievements somehow turn into Luffy's intimidation factor.

And so, the entire world of pirates takes a bizarre turn.

Xiu En slacks off at Marine Headquarters, but Sengoku forcibly promotes him:

"Xiu En, your little brother defeated a Shichibukai. You must have been plotting behind the scenes, right? You're the only one fit for the position of Admiral!"

Xiu En: ???

At Impel Down, Xiu En casually subdued several legendary enemies, planning to clock out on time.

The next day, Morgans published a world-shaking headline:

"Straw Hat Luffy conquers Impel Down overnight! His ranks are packed with powerful fighters, his bounty has soared to three billion, and he has risen to become the Fifth Emperor of the sea!"

Luffy: ???

Then came the Paramount War.

When Xiu En, wearing an Admiral's coat, stood in front of the Execution Platform, he said to Luffy, "If you don't retire soon, I'll end up pushing you into the position of Pirate King!"

The whole world went mad: So Straw Hat Luffy really is a Marines undercover agent?!