One Piece: These Marines Are Trouble, They Play Dirty

[Marines story + mad and unhinged protagonist + modernized development of the Rumble-Rumble Fruit + no female lead + building up a faction] Brian transmigrates into the world of One Piece. Famous throughout the West Blue for being a nutcase who acts on a whim, he becomes one of Zephyr’s students and a peer of Kuzan (Aokiji). Later, after flirting with a female instructor, he is expelled from the Marines’ officer training camp and demoted to a junior major in the South Blue. Zephyr: Brian is the hardest student I’ve ever had to describe. Kuzan: Brian’s got something wrong with him, but he’s definitely strong and a good Marine. Sakazuki: Brian’s an even more infamous lunatic than I am, but he has his own sense of justice. Borsalino: Brian really is nuts, and he’s a real pain to deal with.