Bourbon

A dark skinned light haired man makes an appearance at a case that Conan, Ran, and Kogoro get involved with. His name is Amuro Touru, and he claims that he is a detective. He says that he wants to be Kogoro's apprentice and go on cases with him. So he got a job at Poirot, the restaurant under the Mouri Detective Agency.
This is Bourbon. He has suspicion that Kogoro may have a connection to Sherry because of the case involving Rena Higashi.

In a following case, when Conan gets kidnapped, and Sera, Subaru (Akai) and Amuro(Bourbon) are helping Agasa, Haibara, Kogoro and Ran, Subaru hangs out of Hakase's car and sees Amuro who's driving in the car next to them with Kogoro and Ran. Amuro also sees him, and they both look at each other. (Akai knows that Amuro is Bourbon but Bourbon doesn't recognize Akai.)
Amuro also sees Sera when she saves Conan, and then he thinks to himself "Why is she here..."
So, he must know Sera from his connection with Akai.

As Subaru drives off with Haibara and Hakase in his car, he puts an ear piece in. At that point, Amuro gets a phone call and Sera turns on an ear piece she has in her ear as well. Then we see Vermouth talking on the phone saying, "Anyway, it seems we can trust you...You're going to keep your promise to me right? Bourbon?". So, it is set up to make it seem as though bourbon could be any of those three, but she is in fact talking to Amuro. Subaru is almost definitely listening in on that call, and Sera probably is too. (The promise being that he'll help with Sherry.)

Sera, while walking home with Ran and Sonoko, sees the man who seems to be Akai Shuichi, and runs after him. This is Vermouth disguised as Akai, because Bourbon wants to see if Akai is really dead, by seeing the reactions of people close to him(Akai).

As cases continue, Bourbon occasionally offers his assistance. However, he is really trying to use Kogoro, since he is a detective, to gain leads on where Sherry may be. (Gin knows there may be some connections to Beika, since the case with Rena that occurred there.)

On a later case taking place during a camping trip the Detective league takes in the mountains, Haibara ends up having to take the temporary antidote for the APTX poison in order to save Genta, Mitsuhiko, and Ayumi. The kids take a picture of her in her adult form, because they want to show it to Detective Mouri and have him find her so they can thank her for saving them.
This wouldn't be a problem except, she was wearing a ring the the kids had received as a pass to board a train for a special mystery tour in a few weeks. So, because of that, she can be seen wearing it in the photo the kids took of her as Sherry as well. The kids send the photo to Kogoro, and Bourbon sees it on his computer.
So, the black organization plans to capture Sherry on that train, and kill her.