I'm considering the possibility that Natsuki Koshimizu are the same person However That doesn't look like it works...
Why?
Firstly, She is a detective who has met Conan. At the end of the episode (497) It seems she also showed interest in Conan's character.
They both shared similar eye color and when Ran saw here In episode 479, It was on a boat so her thinking of waves makes sense (Chapter )
Since She's also a detective, it's not unreasonable to think that her involvement with the Mystery girl is apart of a request. As for the request, it could be a search for leads on Akai or to investigate things related to the organization
considering the fact that any connections to the organization and Akai both have to do with Conan and Haibara. Assuming the one who is behind Sera is That Mystery girl and their relations are close enough, you can also see why
they want to find out if they can trust conan by leaving clues without revealing too much (Ch 879). Concerning the relation ship with Akai, I think Akai does have a sibling, being the Mystery person however, The fact that Sera is
Akai's sister has not been truly confirmed. The reasoning behind the bags and black hair is to mislead people into thinking she's Akai's sister.
Things to break this theory?
1) Train Incident, Subconciously calls Akai Shuu-nii (Meaning relatively close however doesn't exactly mean they're related either)
2)The tooth that shows when she smiles (Not on Natsuki but then I haven't seen her smile with a closed mouth)
3)Haven't seen any piercings on Sera (Natsuki does have piercings however can be hidden with makeup)
4)Nakamura pulls her face and no mask comes off (however if makeup is only applied a little make up, both sera and Natsuki can look alike considering they have similar features)
5)Chapter 845- Uses Akai's line with ("... This is our territory...") with Ran (although it's possible that she's using info the mystery kid gave to thoroughly play Akai's sister)
6)It would be rather difficult to play a role of someone's sister constantly so it's not the smartest plan and would likely not be used
7)Sera was in America until recently