One Piece: From Ninja To Pirates
by LegendaryTL
Summary
Meiser, a mixed-blood descendant of the Uchiha and Senju clans, rose to prominence during the Second Shinobi World War. While on a mission to the Land of Water, he unexpectedly crossed over into the world of One Piece and encountered the sea train heading to Enies Lobby! On this speeding train, Meiser met the boy who carried fate on his shoulders.
Luffy: You're a ninja?
Meiser: Yes.
Luffy: Join my crew!
Meiser: Sure... Since I'm here, I might as well settle in. Invited by Luffy, Meiser chose to join the Straw Hat Pirates and embark on adventures with them. Using his powers, he helped his crewmates rescue Robin and save the ship spirit Merry!
Merry: Meiser, what's that?
Glancing at Moria, Meiser replied: That's a loser who's lost his spirit and only knows how to rely on external things.
...At the Marineford War, a colossal Susanoo wreaked havoc across the battlefield...