There's a lot of speculation about who the unnamed 7th Shichibukai is. Most likely it's a new character. But if it's not, then there's a possibility that it might be this guy
And that guy is...
And that guy is...
None other than Kuro !
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Now you might say that Kuro is a fodder who got beaten by Luffy wayyy back in Part 1, but it's possible that he might have improved a lot over the timeskip. Also in the movie Z, it's said that somebody cut off Zephy'r arm and became Shichibukai. Only problem is Kuro's past appearance which has pathetic. But people can get much stronger with proper training. Luffy is certainly Shichibukai level as of now (thanks to Rayleigh's training). Maybe somebody assisted Kuro as well? Who knows? Lol