I really can't think of it. It can't be the Marines, it can't be somebody from before Punk Hazard, chances of it being Monet or Vergo are just crazy low... who's left that would actually be on the ship? Is it gonna be ANOTHER new person?

"No no no.. this simply won't do."

That's all the hint we have so far.
My guess is that it's another "fairy" trying to steal stuff. A cohort of the one Zoro is dealing with.