Tatsu is the true antagonist of the Ocean's Dream filler arc (based off the video game of the same name) of the anime One Piece.
Tatsu first appeared to be an instument played by Noko/Dream that was used to steal people's memories. However, towards the final segment of the arc, it is revealed that Tatsu was the real mastermind behind the theft of memories and that he was using Noko for his Devil Fruit powers (the ability to put people to sleep). On his own, Tatsu could easily be defeated by any member of the Straw Hat pirates (though Zoro was the one consistently attacking it due to it being "annoying"), but when using his powers, he shows a streak of cruelty by showing mirages of the Straw Hats' loved ones (among them including Bellemere, Dr. Hiluluk, Kuina, Kaya, Zeff, Ace, and Shanks), then begins to absorb even more memories in the form of their past enemies (among them Sir Crocodile, Enel, Don Krieg, Captain Kuro, and Arlong). He stole memories in hopes of becoming a Millenium Dragon, but was defeated by Luffy before it could fly away.