One Piece: I'm in Impel Down, and Luffy Is My Brother

He transmigrates into the world of Pirate King and becomes the eldest brother, sharing only one parent each with Luffy and Ace.

Looking for a place that provides food and lodging without overtime, where he can slack off, Carl dives headfirst into Impel Down, reputed to be an impregnable fortress, and takes a job as a jailer.

He awakens a check-in System. In his first year, he receives [Illusion]; in his third year, he receives [Limitless Technique]...

And so, Impel Down gets a deputy warden who works nine to five. The legendary big shots on Level Six become punching bags for him to practice on whenever he's bored.

Then the Paramount War breaks out.

Sengoku issues a kill-on-sight order: eliminate the Straw Hat Pirates who have broken into the prison!

Broadcast live to the world through countless Den Den Mushi, Carl sighs and cleaves the gates of Impel Down in two. “Don't be afraid, Luffy. Big Brother is taking you home. As for the Marines... sorry, my shift's over!”

One strike of [Great Crimson Lotus Ice Wheel Pillar] freezes thousands of miles, and the whole world goes mad!